SOLVED!
The short answer is, take it to an AT&T store!
The long answer is this...Keep in mind, this is my first cellular watch. So, I went to my local AT&T store and explained my issue to one of the reps. She immediately was like, yep, I can help! So basically according to her, AT&T doesn't allow a new line to be added online, for Apple watches unless the watch was bought through AT&T. That's because for watches, it's just like adding a new line, but that new line is virtually tied to your phone's, phone number. It's different than just adding a new line alone. For fraud protection, now, if you're trying to add a watch to an account, it will probably have to be done in-store.
Couple of things to mention...
If you have the same issue and go to a store, bring your watch box (the white box)...Because it will make it faster for them, because they can scan the watch ID and info, directly from the box, with their scanners. Someone else mentioned this in a post I found and the lady that helped me was like, thank your for bringing the box so I don't have to type all these numbers in! They need to scan the info, to tie the watch to the new line they create.
Once they create the new line, you will have to un-pair and re-pair the watch to the phone. That will wipe your watch...so be prepared for that. I did that earlier this morning, trying to sort this myself, so I had a fresh backup to restore from while at AT&T. I waited at the store for it to complete. Repaird the watch to my phone...and then I was good to go. To make sure everything was working, she had me turn off my phone, then called my cell number, and my watch starting ringing...