How do I fix 'No service' and 'Phone not allowed' on my iPhone 11?

Hello, a few months ago I was walking outside, using my mobile data when my phone randomly turned off. I turned it on and found that my phone didn’t have service. This wasn’t the first time that happened so I didn’t panic and shut it down, turned it on and expected it to be back to normal, as that’s what fixed it last time. But my phone still didn’t have service. I tried switching sim cards, the one I have works in other phones too btw, I tried resetting networks and nothing works. I can connect to wifi, but I can’t use my sim card. I’ve went through all settings and it does say in Physical sim >> Network >> Phone not allowed, plus my coverage is expired, but as far as I searched, that’s not part of the problem. Please help me, I have the Iphone 11 and I don’t know if I can repair it or if I have to buy a new one.

[Re-Titled by Moderator]