iMessage will not activate after transferring from iPhone8 Plus to iPhone8 Plus. Help!

Messages Troubleshooting.   

Do you have access to another WiFi network where you can try? That has worked for several people.

Try a restart.

Restart your iPhone, iPad, or iPod touch         

Try a forced restart.

Reset Device.      

If necessary

Hard reset.        

Do a backup and try a factory reset. Then restore from your backup.

Restore to Factory setting.      

Backup - Restore your device from an iCloud or iTunes backup.      

See If you get an error when trying to activate iMessage or FaceTime https://support.apple.com/en-us/HT201422