What would cause a dramatic change for the worse in cell service availability given that the following have remained unchanged?:
Inquiring minds want to know…
Define “change”… number of bars? battery longevity? quality of connection and sound? up/download speed?
No bars and no service in places that absolutely had service just last month.
You’re getting bad reception everywhere and not just in one particular area? It could be a bad RF chip in your phone.
Do other people on AT&T find the same thing at this location? If not then yes, phone is faulty.
Towers in your area are not owned by AT&T but leased, and the lease/support agreement expired?
Possibly the towers, they may be changing frequencies to ones you do not have on your phone, but that one is pretty modern.
Perhaps a full reset of the phone and or a iOS full reinstall may help.
Maybe a corrupted update of the frequencies was sent to you phone or got corrupted on your phone.
You might have switched a setting on your phone to off, such as allow voice over LTE.
Your phone’s hardware may have bit the dust.
Have you tried rebooting it?