iMessage
Cannot sign into my appleid to turn on iMessage on my iPhone
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What happens when you try? Error message(s)?
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.
If necessary
Do a backup and try a factory reset. Then restore from your backup.
Backup - Restore your device from an iCloud or iTunes backup