Sorry, about your statement I am confused. I would like to know what are you trying to do with the RUT950. What I understand is you are trying to have an L2TP connection via the mobile network and it is not established when you are connecting to WLAN? WLAN as WAN source or when you are connecting a device to the router WiFi SSID.
Kindly clarify the things you are doing so that I can assist you properly with the issue you are having.