Syncing iPhone and iPad for iMessage

See if anything here helps.

iPhone, iPad, iPod touch, and Mac using Continuity - Connect.    

How to forward text messages on your iPhone.   

SMS Relay - Set up and use –Yosemite/ iOS 8.    

SMS Relay - Set up and use –Yosemite/ iOS 8  (2).   

Text messages forwarding.    

See these documents.

How to set up and activate iMessage for iPhone and iPad https://www.imore.com/how-to-activate-imessage-iphone-ipad

If you can't sign in to your iMessage account or FaceTime on your Mac  https://support.apple.com/en-us/HT204408

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

Use these support options if you need more help:

Get a call from Apple support to help solve your problem using this link https://getsupport.apple.com/?caller=kbase&PGF=PGF63005&category_id=SC0245&symptom_id=23362 Be patient. Sometimes phone support is not available.

Apple ID Support https://support.apple.com/apple-id

Apple Support Twitter https://twitter.com/AppleSupport  There may be a wait of an hour or so.

Apple ID account security for country-specific and region-specific help

 https://support.apple.com/en-us/HT204169