Contact names and images disappeared from Messages - just number showing. Not due to an update.

I set in motion a migration of my mail from yahoo.com to gmail.com on my computer. Then on my phone I get a notification that my backup needs sorting - can't recall exactly what but I think I checked the boxes that said backup to my Google account.

THEN... I went into my Messages and most of the texts no longer have a name and image associated with them - just a phone number. It's like it has stopped syncing or linking to my Contacts. Some of them are there - older numbers - so I guess that any numbers added since I got this phone 2 years ago have un-linked.

I have checked in my Contacts and on Google and all contacts are fortunately still there, intact, not deleted.

If I create a text message I can select the contact successfully yet, still the associated name does not show next to the message in my inbox.

I have gone into a couple of messages, then into the associated number and it says "connected via Whatsapp". I can then click on a link icon and it brings up the option to connect the number via Google. Once I do this then things are back to normal for that contact - the name/image shows up next to their phone number in the Messages app.

So how do I link my Messages back to my Google contacts rather than just to my Whatsapp contacts?

Thanks for your help, this is making me so stressed out.