Need to upgrade android OS

Nope.   If you are two upgrades behind, your phone is not going to be able to upgrade because those upgrades are no longer available on the AT&T server.

AT&T doesn’t even carry ZTE phones anymore, it’s likely your phone never upgraded past android five anyway.

you should also be aware that when 3G shuts down in your area that phone will be completely defunct. It is not capable of voice over LTE and will not be allowed on the network (won’t work anyway)

Thank you for your assistance.

Looks like I need to buy a new phone.

That's probably not a bad idea in any case.

Thanks again.