This has been a frequent problem for the last 18 months and there are many threads in the forum about it. Just Tap on the tag “offlinebug” Under this topic’s title and you will see many of those discussions.
There are several different things that can cause it. Some are fixable, like improving the mesh in your own house and avoiding the use of most brands of directly connected zigbee smart bulbs. (Tradfri and Sengled are OK, And Cree seem to be OK as long as you have nine or fewer. Otherwise, it’s safest to go with bulbs that connect through a Hue bridge.)
If it’s the hub that’s dropping on and off, it can actually be a problem with your ethernet connection, which again you may be able to fix.
Also, there are no issues with the new app showing things as disconnected, which are just fine in the classic app. So some people find a lot of the problems go away if they use the classic app as much as possible.
But if it’s just a random selection of devices and protocols, it seems to be a platform instability that affects quite a few people at different times. There are a lot of things you can try to fix it, and sometimes they will fix it for a week or two, but then the problem will come back. 
(Whenever someone posts in the forum that they found the fix for their disconnected devices problem and they detail it, I always suggest that they come back in a week and let us know if the problem has recurred or not. Because for many people it’s just an intermittent issue.)
Since everything you described is zigbee and 40 feet away is pushing the boundaries of a single zigbee hop, I would probably start by adding a zigbee repeater halfway between the hub and the room with the disconnected devices, force a zigbee heal, and then see If the problem goes away after a week or so.
But trying to fix this issue is always just trial and error, and you have to give it a few days before you can really be sure whether you are seeing improvement or not.
I know that’s frustrating. It’s one of the main reasons that people will go to a “do it for me“ solution rather than a “DIY“ solution for a Home Automation – – the pro installers have a lot of tools to evaluate the health of a network that smartthings does not provide.