Honeywell wifi thermostat not able to connect

I was having the same thing at my ranch. Only the thermostat and Alexa were connected. Tried connecting with hotspot and that did not work. I ended up setting up a new wireless SSID (called thermostat so i can remember why) and the only thing that's connecting to it is the thermostat. With T-Mobile there's very limited advanced connectivity but I did set it to 2G and WPA2 only, not mixed wpa2 and wpa3).  Those were the only changes and nothing else on this network and it's now working.  

I have a Honeywell RTH6580WF1001 Thermostat that used to work with a CenturyTel router.. I've noted that my Tmobile router has a 192.168.12.1 gateway address. When I redid the Honeywell WIFI setup the Honeywell Thermostat had an address of 192.168.1.1 and a gateway of 192.168.1.1. I assume my problem is the different gateways. Is there any way to change the Honeywell gateway to 192.168.12.1 to match the Tmobile rouuter?

Of course T-Mobile support points their finger at the device and not the gateway.  I tried several iterations of WiFi networks within the T-Mobile gateway, but could not connect my EcoNet device.  I have a cheap Wavlink extender for my backyard that connects to the T-Mobile gateway that my EcoNet can see.  EcoNet connects fine to this extender.  My T-Mobile service is great, router is quite poor in options, local connectivity, and standards.  Likely, I will ultimately connect a real WiFi router to the ethernet port of the T-Mobile gateway and shutdown the WiFi on the T-Mobile.  Can use conventional WiFi router and extenders or spend more and put in mesh network routers.

You mentioned that it was working before so that would mean something changed to create this problem. Since TMO has not issued a new firmware in the past couple months, that can't be the cause. The only thing that likely changed on your TMO gateway is the public IP.

I would contact Honeywell again and tell them the following:

• TMO gateway will frequently change it’s public IP just like cell phones do
• The TMO public IPs frequently show geolocations that are no where near the physical location just like cell phones. Like a couple hundred miles away. As I type this, my public IP says my location is 180 miles south of my gateway.

If either of these cause problems with Honeywell, it could explain why it no longer works since Honeywell services are expecting your thermostat to have the same IP and/or same geolocation when you initially set it up.

You could test this theory by removing your thermostat from Honeywell app and then factory reset to do setup all over again. I'm not sure what all you lose by doing that and keep in mind, all that will do is confirm this theory.

You should also know, if it turns out this is the problem, there is nothing TMO can do about it right now so you may not even have a solution unless Honeywell can make it work.

Same issue.  Won't connect with Honeywell. 

Tried a Honeywell RTH9585WF1004 and same problem.  Then tried an Amazon Smart Thermostat and it worked.  Ironically the Amazon thermostat is made with Honeywell Home technology.  I was worried it would fail for that reason but was very happy it worked.

Thank you for the tip.  Yes, I have tried many different SSID names and security protocols.  I will try other wifi thermostats.  

Make sure your 2.4 and 5 SSIDs are different. That's the only thing I can think of. I don't have Honeywell, but my Nests have no problem.