Device status issues: All devices' status constantly changing and ADT hub always "Loading"

I have two Smartthings hubs: The ADT/Smartthings Security panel, and a v3 Smartthings hub. I have dual-branded sensors running off the ADT panel, and everything else connected to the v3 hub. My wife and I each have iPhones running the new Smartthings app.

For the past 2 weeks, the status for every device excluding the ADT hub has been cycling between the “No network connection”, “Checking Status”, and the correct device status. It stays on each step 1-3 seconds before switching to another. For the ADT Home Security hub, I’m unable to arm/disarm from the app because the status is always “Loading” and doesn’t give the option to do anything.

On a normal device, for example a light switch, I can attempt to hit the power button at the right time and get the device to actually turn on or off, but it sometimes takes a few attempts. The status is sometimes more stable if I disconnect from my home wifi so that I’m accessing Smartthings from the outside world, but it’s not a guarantee.

Interestingly, the integration with Alexa still works flawlessly, and I can change status of any device, or arm my security system through my Amazon Echos.

The issue is solely confined to the Smartthings app, but it’s affecting both phones.

I’ve rebooted all hubs (disconnected power for 4+ minutes, even taking the battery out of the ADT hub), and uninstalled/reinstalled the app on my phone, and nothing has changed.

I did update my router software about two weeks ago, but I don’t remember if that correlated with the start of the issue or if I’m grasping at straws. I’ve never configured anything special for Smartthings so I don’t know why this would affect it.

Can anyone offer any ideas on what I should look for or investigate? I called support but the level one was unable to help and logged a ticket for which I should be receiving a response in 7-10 days. If someone has ideas in the meantime, I would appreciate it.

I’ve resolved this issue finally, after 6 weeks. Smartthings support was terrible; I waited about 2-3 weeks between each reply from them. Before attempting their drastic second suggestion (deleting all devices off the v3 hub and removing it entirely), I reviewed all the settings pages in my router and found Universal Plug and Play (uPNP) was disabled. I enabled that, and everything began working again.

Somehow this must have been disabled during a router update.