Smart phone help please: Currently have Android 5.1 on my phone

[steve2470]

I don't need anything fancy on my phone or a fast phone, etc etc

I browse a few websites when I'm waiting to see the doctor. I make a few phone calls and send a few texts.

The problem is, my current phone is "stuck" on Android 5.1. I have done a few hours of research on Google, and it appears there is no way for me to upgrade my phone past 5.1. The ONLY way is for me to go to AT&T and use the identical phone and that's just silly, IMHO.
My phone is an ZTE ZMax 2 (and yes I know all about the Chinese, not worried, not in the government or military, etc).

The only reason this is even an issue is because Pizza Hut will not support 5.1 any more. I use their app occasionally. Now I can no longer use it on my phone.

I realize, eventually, that probably ALL third-party app makers will abandon 5.1. Android is up to 9.0 now.

Feedback sought, please. Thanks in advance.