Setting up Apple Watch Family Setup for a child in the UK using BetterRoaming

I have purchased an Apple Watch for my child. Added it on my iPhone in the UK with family set up. My iphone is on o2 and I have purchased a contract with BetterRoaming for my child's watch. However my child can not send/receive texts or call. Has anyone managed to get it to work?

[Re-Titled by Moderator]

Original Title: Have you successfully set up an apple watch for your child with family set up in the UK? with which network?