3g coverage change?

My husband and I have been ATT customers for 15 years. Our phones have ALWAYS worked inside our home, which is why we carry ATT for cell service, since many others for some reason cannot get a signal INSIDE our home. On Dec 30, 2019 our devices worked fine. On the morning of Dec 31, 2019, suddenly both of our flip phones on 3G networks have no service. My husband happened to leave for a while mid day, and he got service. When he got back, the service went back to "unavailable" while inside our home and all around our property. Meanwhile, I called ATT support, who had me remove my sim card, etc. Mine can't get a signal, and can't even connect to update the date/time correctly. I know 3G is on its way out, but I can't find ANYTHING that says ATT changed their 3G network/support. But clearly, they have. Has anyone else experienced this? Our phones are searching for a signal and wearing down their battery to nothing in less than 24 hours. We will be forced to either upgrade to 4G (which will knock us off our affordable flip phone plan for the 4 lines we carry) or move to another provider. It really aggravates me that ATT says nothing has changed....it must be my sim card, when clearly something has changed since both my husband's and my device suddenly can't find service.