There is a USB port in the router and there is a page for 3G/4G on the router setup page - so one would assume you could buy a 4G dongle and have it work as backup should your V/ADSL fail.
However I have not seen any manuals for this router and I have not heard of anybody even trying it out. Let us know how you get on if you do try it out.
Judging by the amount of times some peoples routers go down in this forum it might prove cheaper for you to have a second supplier with a second line and another router as your backup ? Just a thought if it proves a really expensive backup and it is critical you have internet. Whhen I NEEDED internet I had two lines, two suppliers and it worked great.
Also your question re should you lose wifi depends on why your internet connection is going down, if it is the router rebooting you will lose WiFi and if it is really regular - maybe try disabling uPNP and any other service you do not need.
If it is just the line dropping then I would expect your WiFi connection to stay up, but frankly I would not be suprised with anything with these routers they are throwing at us.
