We have found 4 possible solutions to your problem.
Before you start this troubleshooting guide you need to check one important thing:
Can you make a voice call?
Yes
No
Why do we ask this question?
It's important to find out if you can make a call. If you can't make a call, you need to solve this problem first. When the problem is solved, you will most likely also be able to send and receive iMessages.