I have a very unreliable ADSL connection (in central London if you can believe it) that is constantly dropping. The Vigor has one of the better ADSL chipsets out there but even it can't work around BT's ineptness and failure to understand that "noise on the line" means "something needs fixing". As a result, I have purchased a 3G USB dongle which I am trying to use as a backup.

The Vigor 2820N (running latest firmware) allows 2 WAN connections and will activate the 3G connection when the ADSL goes down, which is exactly what I'm looking for. However there are some problems with my setup that I hope someone can assist me with.

The ADSL WAN has a fixed IP from my ISP and static DNS servers. As I use a static VPN, these DNS servers are actually inside my company network. The VPN is a point-to-point, IPSec tunnel to a PIX 515E and depends on having a static IP for authentication.

The 3G connection is from Three and is completely dynamic for both WAN IP and DNS servers; it gets this from DHCP when it connects via PPP.

Although the Vigor has an internal DNS forwarding server, its DHCP server hands these out to DHCP clients. This is the core of my issue.

I see two solutions:

1. Set up a VPN that the 3G connection can dial into. Since the PIX depends on having a static IP, I'm not sure how to do this - Three is unlikely to give me a static IP.

2. Configure the Vigor to hand out its own IP as the DNS server, and then forward requests to the appropriate DNS server on the connection which is active. I would of course lose connectivity to my office LAN during ADSL outages but that's better than nothing.

Any advice? Much appreciated!