GL-M2 board - Cellular connection established but no internet

I still struggle with the GLM2 as it appears to work, is shown as online but not data is transmitted.
I set the Mult-WAN mode to failover mode with WAN (V-DSL) in 1st place and WWAN (Cellular) in 2nd place . However when I cut the WAN line to simulate a breakdown of DSL nothing seems to happen, so no switch to WWAN and no internet at all. Sometimes WWAN breraks down simultaneuously when WAN is stopped. Also I discovered the same situation when I try to use tethering with my iPhone. Everything is shown as connected but no internet until I reconnect the Ethernet connection. Any idea what could be the reason?

How do you simulate a DSL failure? unplug the WAN cable? Or unplug the ISP cable of the DSL device (DSL device power on but the drop the ISP connection)?

Please SSH to the router, and check the WAN link from the kmwan log:
echo 3 > /proc/gl-kmwan/debug; logread -f