Simply put, AT&T is abusing its bring-your-own-device (BYOD) program to force people into 5G. Have a working and compatible 4G phone? You can pound sand, buddy.
I have Consumer Cellular (uses AT&T network), and it's been really acting up lately. My phone has 3G and LTE. Sometimes I have to switch to 3G for it to work. I think I have to buy a 5G phone for it to work right. It's only like a 2 year old phone and was $250 I think (Nokia 4.2 unlocked).