I jumped online tonight thinking I would find a solution, but it appears this has been a problem for quite sometime, what a disappointment! I too have trouble with the new app, it seems far more flaky than the classic, particularly when it comes to adding new devices.
But right now my problem is that about 3 of my 10 sensors are “offline” and the idea of going around and pulling batteries is a pain-in-the-axx. I can’t believe that is the only solution Samsung can come up with.
Does anyone from SmartThings actually monitor these forums? This issue with sensors has been around for 3 years according to this forum, and the third post says this has been a problem for a long time (in Jan 2017).
Being forced to use an app that removes the functionality of the previous version is bad enough, but these sensors are SmartThings branded. If their own devices don’t work with it, then what hope is there for anything else?
Removing and reinserting the battery doesn’t work. Resetting it also doesn’t work. Even if they did, it would completely defeat the purpose of having a sensor if you have to stand next to it and switch it on and off all the time.
I have given up on the sensor in the garage. It’s now permanently offline (there is a mesh network device in the garage and everything else, the door openers, the lights etc all work fine). The one at the other end of the network, in my office, is also permanently offline. I can reset it if I take it into the house and stand by the hub. If I leave it in the kitchen it will work, but as soon as I take it back to the office it goes offline again. As with the garage, there is a mesh device in the office (on the same shelf, approx. 50cm away from the sensor) and the smart plugs in there all work perfectly. The wifi signal is, as you’d expect, excellent.
Clearly it’s a distance issue, but since the office sensor is about 6m away from the one in the kitchen it is ridiculous that it doesn’t work. If the cheap Meross plug can stay connected and operates on a timer setting from SmartThings, why on earth can’t the SmartThings branded device stay connected?
Evidently, Samsung are very invested in selling this, but much less invested in actually making it work.
I would love to replace every light switch and plug socket in my house with smart versions, but if I can’t rely on the sensors to actually stay connected, what’s the point?
Over the last 18 months or so the investment by Samsung has dropped off a cliff. It seems the policy is “it sort of works, so we’ll let the community figure it out from there”. @JDRoberts is extremely helpful and knowledgeable, and without his advice I would have given up on this years ago, but that should not be the way this works. Samsung product, Samsung problem.
Thanks for the shout. I’m sorry you’ve run into such a frustrating issue. 
This forum was set up many years ago so customers could help other customers, mostly with ideas and custom code. It’s never been an official support channel, although there are a few employees who post here from time to time.
Officially, they monitor their twitter feed, Facebook page, app reviews, and the official support channels. So if you want to Get company attention, you need to use one of those.
There are two completely separate possible causes, one we can Maybe help With, one we can’t.
Building a Strong Mesh
First: those sensors use Zigbee. Zigbee has the shortest transmission range of any of the networks that SmartThings support. in the smartthings hub is a plastic box that includes several different radios, zigbee, Z wave, maybe Wi-Fi. Each is essentially participating in a completely separate network. Zwave repeats only for zwave, Zigbee repeats only for Zigbee. So it’s not surprising that the Meross plugs, which is Wi-Fi, might perform differently than the zigbee sensors.
How many, if any, Zigbee mains-powered devices do you have? It may be that you just need a few more repeating devices around the house. (The IKEA Tradfri plug in pocketsocket works well for this, and costs under $10 when bought directly from IKEA.)
See the following thread for explanation. Start with post 11 in that thread, read it, then go up to the top of the thread and read the whole thing. (The topic title is a clickable link.)
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OK, that’s the problem we might be able to help with. Now onto the next.
SmartThings Device Health
Smartthings as its own “device health“ Value which is independent of the protocol being used. It’s based on how long it’s been since the device has checked in. But the problem is that smartthings is mostly a cloud-based system, this has been a cloud-based utility, and lots of things can introduce enough lag that a device gets marked as off-line even when it isn’t. This has been a problem since device health was introduced and there’s nothing we as customers can do about it.
It appears from some employee comments that there is a long-term plan to move the device health check from the cloud to the hub itself, which should help. But no timeline for when this will actually happen.
These Sensors
I agree absolutely that since these are Samsung branded devices, Samsung should support them. So I would go ahead and put in an official support request through the link in my previous post. But be aware that the first people you talk to you are Samsung first line support who are just working from a database, probably know very little about smartthings, and are just working off Scripts. So it’s very likely that they will tell you to reboot and reset a bunch of other tedious things that likely have very little to do with the actual problem. 
It’s up to you wherher you want to actually do any of what they suggest, but at least reporting it there does get it logged into their own statistics. Which posting here does not.
Thanks @JDRoberts, helpful and practical as always. I’ve ordered a Tradfri motion sensor, signal repeater and wireless control outlet. I’m going to see which of these is best. My wife hates the plug in things so if the repeater works well, she’ll put up with a USB port plugged in behind sofas.
I have logged the issue with Samsung, but since everyone is using COVID as an excuse to offer terrible levels of service now, I’m not hopeful.
In most cases, only Mains powered devices can act as repeaters, so the Ikea “repeater“ and the pocketsocket will perform this function, but the battery-operated motion sensor will not.
Also, in order to get Zigbee devices which are already on your network to use a new device as a repeater, you have to do a “Zigbee heal.” This is easy. After adding all the devices, just unplug the hub from power (including removing its batteries if it has any) and leave it off power for about 20 minutes while leaving all of the other Zigbee devices on power.
This will cause the other devices to go into “panic mode“ because they can’t reach the hub to check in.
Then when you put the hub back on power, all the other devices will try to find the most efficient path back to it which will cause them to consider the new repeaters as potential “parents.“ Which is a good thing. 
All, have had these sensors for a few years and struggled with the same issues and tried a variety of the remediations to little help. I made another run at things a few weeks back and discovered that there was a new firmware for these devices and by default the firmware upgrade for zigbee devices was disabled by default in my account. I reconfigured to allow upgrades and after about 24hrs all my devices completed the update (it only allows five at a time so it can take awhile). It has been two weeks since that and I have had zero issues with the devices going offline.
If you haven’t already done this then I highly recommend upgrading the firmware on these devices as for me, I finally seem to have a stable environment after two plus years.