wifi won't connect to honeywell thermostat 900

I have a honeywell thermostat 900 which is connected to my wifi.  Comcast upgraded my system and installed a new modem. Since then I am not able to connect to my thermostat.  I have talked to Comcast several times and even had a knowledgeable comcast tech come to the house.  I have had many suggestions about a solution from both comcast and Honeywell.  I tried them all but nothing has worked.1