Did not find gl_monitor in the list, only carrier-monitor.
Now I also added a default route (System/Static Routes) which works (checked by using Network/Diagnostics/TraceRoute) but only with IP addresses, not with DNS names.
Without the static route, all pings get lost.
So the interface configuration seems to be ignored (I have set the default gateway in ‘General Settings’, checked ‘Use deafult gateway’ and configured ‘Use custom DNS server’ in ‘Advanced Settings’).