Honeywell thermostat disconnected from wifi, won't reconnect

Two days ago my Honeywell RTH6580 thermostat lost connection to wifi.  I went through all the steps, including with assistance of Resideo (Honeywell) support to try to reconnect.  They said I need to contact Xfinity so here I am.

From other reading online I gather it may have to do with Xfinity having combined 2.4G and 5G and this somehow makes the thermostat unable to connect since it only works with 2.4G.  Regardless, I need help.

My modem is a Motorola MT8733 Docsis 3.1 Modem + AX6000 router + voice.  

(If I try to select tags for this, the screen hangs and I have to go back and retype this entire message.)