My few problems improved when I altered the signal strength. With Hue, Starling, and eufy bridges within an Asus AiMesh household, my Hubitat C8 came set at a power of 16 by default. Lowering it to 12 made my system more stable. Haven't had a need to test lowering it further.
I know there no one remotely nearby my home, which could introduce signal/broadcast conflicts. If I had neighbors close enough, that could throw add'l potential causes into the pot. I'm just trying to appreciate why your issue is sudden. . . anyways, try altering your Zigbee power, see what that does for you.
Odd - So it was extremely reliable for about a year. I then added to more sonoff themorstats - then all of a sudden all my zigbee devices stopped talking to the hubitat? Is that usually how a lack of repeaters manifests?
Yes, those Innr SP 224 plugs are Zigbee repeaters. I have 2 of them and they work pretty well on my mesh as repeaters. If they stopped talking to Hubitat, the first thing I would try is to unplug and then plug them in again.
Why not remove them, get the other stuff up and running, power cycling the plugs.
The thermostat is battery powered?
Maybe one of the repeaters crapped out and threw the mesh for a loop.
I know my 3rd Reality temp/humidity sensor wouldn't connect to the hub when I removed a repeater plug. I had to bring it upstairs and re-pair, or maybe just pushed the button on it, or maybe just nothing, I forget.
Does C7 do the graph? If so http://your-hub's-ip/hub/zigbeeGraph , a link on the Zigbee Details page.
Maybe some of the other tools on the Zigbee Details page could help, if available on C7. You could see when they last communicated with the hub, if and when they drop. Starting up the zigbee log could get you signal strengths.
I don't think that adding 2 Sonoff TH sensors can destabilize a Zigbee network.
I was about to say that a new WiFi interference may be the reason for the sudden drop in connectivity reliability, but the OP has already tried changing the Zigbee channel.
No responses registered in the live logs for the Sonoff TH sensor is normal - this is a deep-sleeping Zigbee device, and it does not respond to configure/update commands except when it wakes up, sending a temperature or humidity reading update.
In the past, I have seen the Sonoff Temp/Humidity sensors create mesh problems. Some sort of error during the joining process will identify them as routing devices, which they are not, and the mesh becomes unstable before they eventually stop communicating.
Check the zigbee routing table, xxx.xxx.x.x/hub/zigbee/getChildandRouteInfo, and make sure they are not in the neighbor table. If they are, delete and re-pair them.
Agreed. The information from which that graph is made permits the creation of a pretty picture, but it has limited utility. There's much more to be gained by using an Xbee3 while setting up a zigbee mesh.