Adding a line to a grandfathered plan?

I am on a grandfathered plan, "The new Verizon Plan Unlimited." I was at the max amount of devices for my plan, but wanted to add another phone, so I canceled the data on one of my tablets. Now I am being told I cant add another phone to my plan without upgrading to a new plan by the online chat. Is this true? I was able to add a new line a few years ago.

Thanks