@lotharjk 

Be sure the SSID is not hidden in the settings as that sometimes is the issue.

See https://www.honeywellhome.com/us/en/support/why-is-my-thermostat-wifi-not-working/ as you may need to re-pair the unit because you changed the router.

@ArcherC8 

@lotharjk

I am having the exact same issue with Deco X5700 after a firmware update from 1.0.5 build 20210420 rel 75992 to 1.1.2 build 20210722 rel 66057.

All of my other devices except the Honeywell T9 thermostat rejoined the network.  It is the only device that has the issue.  The network does not appear in the list on the thermostat, but other networks appear in the list.  It will not let me add the network manually (says network not found).

As far as I can tell there is no way for the Deco to "hide" the SSID, unless that is what the firmware update caused.  None of my other devices (garage door openers, Ring cameras and doorbells, Wemo smart plugs, etc) have been affected by this firmware update.

The easiest thing for me to do would be to revert to the previous firmware version (1.0.5 build 20210420), but Deco will not allow me to do that.

To test and see if it was more than just the Deco having this issue, I found my old Linksys Velop wireless router and plugged it in.  The Linksys wi-fi network showed up as the first available network on the thermostat.  This tells me that the problem is a combination of the Honeywell T9 firmware 01.02.08.00 and the Deco firmware 1.1.2 build 20210722 rel 66057.

I am contacting Honeywell to see what they can do about fixing their firmware, but I think TP-Link ought to do the same and examine what they changed that broke this connectivity.

BTW, I bought the Deco to replace the Linksys after a similar issue with a Linksys that broke connectivity with Wemo.  I may be also ditching the TP-Link Deco if this doesn't get resolved.  At least the Linksys allowed me to downgrade firmware to temporarily resolve the issue until they fixed their firmware.

@Kevin_Z I recently went and got a T-Mobile home wifi to test out for my second home and set the SSID the same as this router and everything paired up with it including the thermostat once i powered down the AX90. I just recently reverted back to the AX90 and once again the thermostat is the only thing not seeing the 2.4 network SSID. The thermostat see's every one else networks in the neighborhood except mine. I just got done upgrading the firmware again to the latest build that just came out and it still doesn't see the network SSID. I currently have 2 other devices contected to the 2.4 channel. So the channel works and my thermostat will connect to other routers. Something must have changed in the broadcast from the AX90 for the honeywell thermostat not to see the SSID from it. Will resetting to factory defaults revert the firmware or do I have to manually plug into it to do that?

@lotharjk 

Since it is only one device that is having an issue, then I would suspect the device a bit more than the router.

Since it worked before the firmware upgrade, I would check the WIFI settings for your 2.4 WIFI and see if there may be something causing the incompatibility. For example, your thermostat needs an 802.11n signal and if you have the router's WIFI setting to use AX only it would not connect. Security may be another thing to play with. Try specific channels and widths. 

Another thing that happens to some devices is when you change routers using the same WIFI SSID, you have to delete (forget) the network setting on the device and re-establish it. Seems to happen more often with Apple devices but others have been reported.

If the thermostat has a factory reset, I would try that also.

Resetting the router to factory will not change the firmware level.   Reverting to a previous level of firmware requires you loading the previous level in a manual process which may vary a bit by model.

@ArcherC8 I've already tried changing the settings on the router multiple times including the signal. I have two different routers that connect to it with the same ssid. I even setup a mobile hotspot with a different ssid and connected to that. I've changed channels and widths. I've pretty much tried every option out there except going back to the old firmware which is what I guess I will have to do. If that fixes it then I will know for sure it was the router firmware that did something.

ArcherC8 wrote

@lotharjk 

 

Since it is only one device that is having an issue, then I would suspect the device a bit more than the router.

 

Since it worked before the firmware upgrade, I would check the WIFI settings for your 2.4 WIFI and see if there may be something causing the incompatibility. For example, your thermostat needs an 802.11n signal and if you have the router's WIFI setting to use AX only it would not connect. Security may be another thing to play with. Try specific channels and widths. 

 

Another thing that happens to some devices is when you change routers using the same WIFI SSID, you have to delete (forget) the network setting on the device and re-establish it. Seems to happen more often with Apple devices but others have been reported.

 

If the thermostat has a factory reset, I would try that also.

 

Resetting the router to factory will not change the firmware level.   Reverting to a previous level of firmware requires you loading the previous level in a manual process which may vary a bit by model.

@lotharjk 

Will it connect to or see your guest network?

Going back in firmware is sometimes needed. I have done it to correct a port forwarding issue.