iMessage won't Activate on iPhone

Hello,

I recently began using an iPhone 7 Plus (running IOS 15.8.4) and a cellular plan (Mint Mobile). So far, i haven't been able to get iMessage to activate. When I go to settings and try to manually activate it both by iCloud email and iPhone, neither works. I have been using iMessage on my Mac with the email, I just can't get it to work on the phone. This is especially difficult because people send messages to me and the system attempts to send an iMessage to me but I never get the message. SMS works like normal. What can I do to fix this issue?

Thanks!