Hello MrXamad,
Thanks for reaching out to the Apple Support Communities. We understand you are experiencing issues with activating iMessage on your iPhone using your new cellular carrier. We are happy to help provide some guidance.
It looks like you posted to the Apple Support Communities from the related article that goes over the steps to take if you aren't able to activate iMessage. Just to confirm some of those steps, is your iPhone able to send and receive SMS messages (regular text messages) as well as international SMS messages? If you aren't sure, you would want to reach out to your cellular provider to ensure your device is capable of this. Being able to send and receive SMS messages and international SMS messages is a requirement for iMessage to activate. You can read that in the section titled "Still not activated after a day":
If you experience an error when trying to activate iMessage or FaceTime
If you continue to experience issues, we would recommend reaching out to Apple Support directly using this link: Get Support
Cheers!