how do i sync text messages across ipad and iphone

If you have an iPhone - and both iPad and iPhone are signed-in to iCloud with the same AppleID - to send and synchronise SMS/MMS messages from your iPad you must explicitly enable Text Message Forwarding for your iPad on your iPhone...

On your iPhone:

Settings > Messages > Text Message Forwarding - set to ON - and explicitly enable your iPad from the list of devices.

Also ensure that Messages in iCloud is enabled on both your iPhone and iPad:

Settings > [Your Name / AppleID] > Messages - set to ON

Go In your iPhone settings go to messages then get in Text messaging forwarding option get there and check if your iPad is listed and the toggle it’s ON if it’s off then turn it ON note that iPhone and iPad must be logged in with the same Apple ID