Can't enable iMessage option in Apple Watch SE (2nd Gen) settings

We purchased an Apple Watch SE for my daughter this year so we could keep in touch with her when she's out (such as if she needs to stay after school or is out with her friends etc). The watch is a stand - alone watch, and is not paired directly with another iPhone (i.e. it has it's own number and line, separate from my account). I do use my phone to setup Schooltime etc (and it was required to use a phone to do the initial setup).

My question is - when I go into the Messages settings on the Apple Watch, the iMessage option is switched off. When I tap on it, it just spins for a few minutes before refreshing and is still in the off position. Is there a reason this is happening? I dug around here in the forums and online, but haven't found any definitive answers for enabling iMessage. The answers I did find are tied to Apple Watches linked to the same number on the accompanying iPhone, not stand alone watches being used for a child. The answers there are usually to turn off iMessage on your phone and then turn it off on the watch, and then turn them on again. However it's never enabled on the watch to begin with, and it's not linked to the same number anyways.

Thanks in advance for any insight anyone has on this!