iMessage activation issues

Can someone please help me... I have tried everything.

I have a iPhone X, 8 plus MacBook Pro 2017, iPad Air 2 and iPhone 5s with iMessage activated fine.

But on my iMac I keep getting the most user friendly exception "An error occurred during activation, Try again"

I have also tried this guide "https://www.imore.com/how-fix-imessage-and-facetime-activation-issues-macos" but it has not worked, I've also installed a clean version of macOS Mojave and the same problem still persists.

This problem came from switching from an old iCloud account to my new one. I have also removed the iMac from the old iCloud account and added to the new one.

Thank you for all help

I