I have a business boardband fibre contract with BT. The BT Business Smart Hub supplied by BT comes with a 4G Hybrid Connect which will use the 4G network in the event of a problem with the fibre. The 4G device is not standalone - it will only work it paired with the BT Business Smart Hub. I prefer the DrayTek router to the BT one, so configure the DrayTek to use the BT system as a Failover. Is this possible? If not, why not? 

If the BT hub has no broadband connection the status like this. I have marked red around where it says there's no broadband, and greed where it says the 4G signal is good. 

https://kirkbymicrowave.co.uk/bt-hub.png

So I have all these bits together. 

https://kirkbymicrowave.co.uk/routers.jpg

Given there is an internet connection on the BT hub, is it not possible to run an ethernet cable from the BT hub, and put that into the DrayTek, so the DrayTek uses that as Failover? If so, how do I configure the DrayTek to use it? I've contacted DrayTek UK support a couple of times, but don't seem to be getting anywhere, with several days between emails being answered. I was told use use WAN 1, but it is not clear to me if WAN 1 is configured as a Failover, which port on the front panel is used for that?  I assume one of the ports has to be dedicated to that role, and it not be a general LAN port.