Failed to activate my new device because I can't download the Xfinity App

I have an old iphone SE. The mobile support representative told me it's compatible with Xfinity, thus I ordered a new mobile line for it. However, it requires the Xfinity App to activate the phone. But the App requires iOS 16.0 or later, my old phone stopped at iOS15. I tried to activate the phone with the Xfinity App on my other phone, it says activated. However, my iphone SE still says "No service" with the new SIM card installed. What should I do?