Thanks Ahmed. The system is "unfortunately" already installed at the customer so I can't play around anymore.
I've reflashed it and just connected to my existing wireless network (via scan) and configured it as client (wan firewall) and wwan interface. This works to get internet to the LAN. So I kept it at this for the moment.
I have also encountered another issue, but will post about it later. Thanks again, when the next device arrives at our office I'll try again.
Just a picture of the RUTX10 at work ;)