New Ipad 7th Gen 2019 10.2" not sending iMessage to one person

Hi there, we just bought an iPad with cellular service for my Mom. I am trying to set it all up to help her, and was sending iMessages to each family member to make sure it was working. There was one phone number that it would not let me send to. It kept saying they weren't registered with iMessage, but that's not true. I send iMessages to her on my iPhone and on my mac. There are no issues, but for some reason this iPad will not let me send to this number. She has an iPhone, iMessage is registered and/or turned on. We've trouble shooted via the internet and tons of different posts, what can we do? Yes this is important as my parents are elderly and live very far away from all of us. The one child she cannot reach is the one that helps the most due to being the closest in proximity (which isn't that close). Please help! We are so excited to have this...but we need it to work properly!