Apple Watch with family setup – which providers support it ?

We’ve bought an apple watch for our son and want to set it up with an esim as part of family setup. According to apple https://www.apple.com/watch/cellular/#table-family-setup this is supported by Swisscom and Sunrise carriers.

I have my abo with Yallo, so I got an additional prepaid there, but I cannot set it up with family, Yallo customer care says they don’t support family setup.

Has anyone gotten such a setup to work and if yes, with which carrier / provider ? I’m a bit confused, I don’t understand whether the network is important or the actual provider, i.e. you are on Sunrise network with both Sunrise and Yallo, and also some other providers. Also not sure if it matters where my own abo ist, on the phone that manages the watch, or just the abo/sim of the apple watch itself.

Thanks for any help!