Hi All, to close it off, I finally got around to it. It does take a bit of work to configure:
the SIM card needs to go into a normal phone first, to deactivate the PIN and enter the activation code when placing the first call (in case of a new SIM card and new number)
the APN (access point name) needs to be cofigured manually on the watch to enable mobile data, the chat function doesn’t work without it, more importantly it seems the location tracking wouldn’t either. I copied the APN settings from my UPC phone.
Now everything is working as it should, locations are shown very accurately, too (Zürich)