Mobile tethering to a Vigor router

In the event of an outage, you can in theory tether your mobile to the router, and then all devices will continue to operate.

I do realise that you could very quickly chomp through a lot of data if not careful.... but handy for small outages like when BT are playing in the local cab and it all stops working.

However, I tried to configure it earlier and it didn't work. Anyone else had more success?
https://www.draytek.co.uk/support/guides/kb-wan-wire...