Carrier Touch thermostat "A" will not connect to Orbi RBR850

TLDR:

Our Carrier Infinity Touch version "A" thermostat will no longer connect to our Orbi RBR850 or its satellite RBS850s after rebooting then resetting them. The thermostat connected to the Orbi Guest network for years.

No changes to either the Orbi or thermostat firmware, which are both up-to-date. I've now spent days trying to get a connection. The best I've gotten is a temporary connection by connecting the thermostat to something else (laptop, another WiFi access point) which has the same network name and password, then turning that off, which then appears to give me a connection to the Orbi system. I'm not even sure that's a real connection, as the Carrier remote app won't work with it, but the thermostat does show up in the Orbi web portal Attached Devices. Thermostat connects when the Orbi has encryption off.

Detail:

We had a lot of trouble connecting our Carrier Infinity “MyTouch” version A thermostat (SYSTXCCITC01-A) to our Orbi RBR850 several years ago. After that it’s been pretty stable. The thermostat has occasionally required a reboot by pulling it off the wall for a few minutes then reinstalling. The thermostat has been on the Orbi’s Guest network.

Recently we had some trouble with at least one satellite, so I did some rebooting, resetting, restoring from settings backup, and finally a full reset of the entire Orbi system with new manually entered settings based on screen shots of old settings from the web interface.

Early on in the rebooting process the Carrier “A” thermostat stopped connecting. I cannot get it to reconnect to the Guest network.

The Carrier A thermostat will connect to:

- laptop sharing Ethernet to WiFi, same network name & password as the Orbi Guest network.

Works both with and without the RBR850 WiFi on, with the laptop close to the thermostat. I discovered that the laptop uses the same router IP address as the Orbi guest network, 192.168.2.1, and assigned an IP address which doesn’t conflict with the other two devices on the Guest network.

- Apple Time Capsule, Ethernet to the RBR850, Orbi WiFi off, same network name & password as Guest network.

It put the thermostat on the main Orbi network, IP 10.0.0.0.x.

For both the laptop and the Time Capsule if I turn them off the thermostat appears to stay connected to the Guest network for a few minutes. The thermostat shows up in the Orbi web interface under “Attached Devices” as a Redpine Signals device. (The thermostat does not show up in Attached Devices while it’s on the laptop or the Time Capsule. The laptop I think is doing double NAT, but the Time Capsule is not.)

The thermostat also connects to:

- iPhone as a hotspot, different network name and password.

- RBR850 Guest network with no encryption.

The Carrier A thermostat will not connect to:

Any combination of the Orbi with encryption, on main network, IoT network, or Guest network, satellites on or off, will not work. The top encryption the thermostat has is WPA2. The bottom encryption the Orbi offers is WPA. Tried both.

I once got a connection to the main Orbi network, but I’m not sure how. Haven’t been able to replicate that, and I want it on the Guest network anyway.

Other notes:

• No changes to either the Orbi or thermostat’s firmware. Both are latest firmware. RBR850 is v7.2.7.15._5.1.16
• The thermostat is 2.4 GHz only.
• WiFi scan with my phone at the thermostat gives RSSI -56 to -62 dBm on the Guest network. Neighbors' are in the -80s.
• The thermostat has no local network control. Remote control is only through Carrier’s server via Carrier’s phone app.
• Carrier claims that their thermostats don’t work well with mesh systems. But it won’t connect when I have only the RBR850 on. And, it will connect when there’s no encryption.
• We also have a Carrier Touch “B” thermostat and a Ting sensor on the Guest network. These reconnect if they or the Orbi system lose power. They (or at least the B thermostat - the Ting is too close to the router) are able to shift from the Orbi to the laptop and back, and from the RBR850 to an RBS850.
• Our two satellite RBS850s are connected to the RBR850 via ethernet. (We have a third RBS850 not currently connected because it shows up as an “Attached Device” instead of a satellite - that’s another issue.)
• The Orbi has these settings in Advanced>Advanced>Wireless in the web interface:
• Enable 20/40 MHz Coexistence is OFF.
• CTS/RTS Threshold (1-2347) is set to 2347.

Carrier suggests I get a cheap TP-Link router, connect it to the Orbi router, and use it’s default settings to connect the thermostat (!). I don’t want to do this because it adds additional WiFi traffic and can’t be put on the Guest network. We have an AT&T gateway. It’s possible I could put the TP-Link on that, or use the gateway’s own WiFi, currently off. But the gateway sits next to the RBR850.

I’ve also thought about trying a WiFi repeater, but it would still add WiFi traffic.

Carrier’s “B” thermostats go for $400-$500 on eBay used. They now have a “C” thermostat, which is closer to $1000 on eBay. Plus a service call to set it up.

Any ideas?