OK - just did some tests to see what happens

[1] WAN cable to active Internet, Plug in mobile via USB & set to Tethering On, Logged into router GUI
Internet shows Cable active (Green dot), also shows Tethering available so hit Connect, Internet now shows both Cable & Tethering active (Green dot on both)
Ran Speedtests which show current speed available is via WAN cable internet connection, Tethering not used even though it shows as also active

  • conclusion, plugging in mobile with tethering does not deactivate the WAN cable if connected to an active internet, WAN cable will continue to be used as the internet connection

[2] WAN cable disconnected, Plug in mobile via USB & set to Tethering On, Logged into router GUI
Internet shows Cable inactive (Grey dot), shows Tethering available so hit Connect, Internet now shows Tethering active (Green dot)
Ran Speedtests which show current speed available is via mobile Tethering internet connection

  • conclusion, plugging in mobile with tethering needs to have the WAN cable disconnected or no WAN internet connection for it to become the active internet connection