Recently my signal strenght has dropped to -122db

Recently my cell service basically went away at my home. Called AT&T support with no help. So they connected me with Don, in Tech support. We discussed problem, he ran diags on ??? and thought all looked OK. Mentioned closest tower to me is approx. 2.5 miles, yet I have to go on my deck to get 1-2 bars. Suggested I get a new SIM card. Card is less than a year old, CAT S60 phone about 18 months old. Phone has all updates. My WiFi is perfect via Optimum cable router, only I can not make or receive phone calls. Figured out how to display signal strength and it varies from -110 to -122 inside, 15' from a huge window and 20' behind me are glass sliding doors.  My wife uses Verizon and that service is fine.  Many discussions have lead me to believe this correlated to AT&T dropping BAND 2. Only band I seem to detect ANYWHERE in my town is BAND 17. What's the deal? Thanks for any suggestions, George