I'm wondering if it's possible to use a wireless connection as a backup connection to the internet from a DrayTek 2920?

I have a Huawei E585 (MiFi) device which I would like to utilise on the odd time that my internet connection goes down. It won't work via USB and it seems to can't get 3G dongles without the credit expiring now

I can get the router to connect to the Wifi Access Point but am not sure how to set it up so that it will route trafic via it when the main WAN1 port goes down.

Any suggestions please?

Thank you very much in advance.