This solved it for me:
You have to make the option Text Message Forwarding appear in Messages iOS Settings. It will only appear if you are signed in with the same apple id address on both of your devices IN iMESSAGES specifically. So, bare with me:
On iPad: Settings>Messages>Send and Receive> check your apple id address.
On Mac: Messages>Preferences> check apple id adress
On iPhone do the same thing just sign out and sign back in with the address that you are signed in with on your iPad and/or Mac. After that i got the Text Message Forwarding option on the iPhone, turned it on, received a code on my Mac, and entered it on my iPhone.
The crucial thing for me was to log in with the exact same adress on every device, and mind that the that you are logged in iMessages can differ from the adress that you used to log into iCloud on your phone, iPad or Mac.
Please tell if it works.