Thanks for your reply.
I got one step further:
Now I can connect from my laptop computer to the Raspberry Pi via VPN using the new IP address assigned by ZeroTier. I can establish an SSH connection, access the UAVcast webinterface and connect Mission Planner via TCP. That works both if my laptop computer is connected via Wifi or its internal SIM card to the internet. Good first step.
Unfortunately I can only establish these connections as long the Raspberry Pi is connected via Wifi. As soon the RPI is only connected via the Huawei E3372h 3G/4G I cannot connect to it anymore, neither via SSH nor UAVcast webinterface nor Mission Planner. The Huawei E3372h 3G/4G shows a cyan solid led, therefore seems to be well connected to a 3G+/LTE network.
When I reestablish wifi I see following network settings in UAVcast:
Via SSH I get following information from ifconfig:
There are several indications that there is traffic going over the Huawei E3372h 3G/4G, but apparently not the traffic needed for UAVcast, SSH and Mission Planner.
How can I get the full connection established when the RPI has only connection via the Huawei E3372h 3G/4G (no wifi)?