Airport Extreme won't connect to router

I have a Speedtouch 510 Router/Modem (ADSL) connected to an Airport Extreme. For over a year it worked fine (except I was having issues with iChat connections). It was recommended to upgrade the firmware on the router from 4.0.0.9.2 to 4.2.7.16.0. (the latest available) This I did and have the confirmation window to prove it. After the upgrade I could no longer connect to the Internet - either with the Airport OR even using the built-in Ethernet connection direct to the router. I called the ISP who set me up to connect to the internet using the built-in ethernet by configuring the network through System Preferences. However, I still cannot connect thru the Airport - and the ISP simply doesn't want to know I have a wireless network OK (because two computers on this network can talk to each other) but I simply cannot connect to the router wirelessly. I tried switching everything off overnight (as receommended in one thread) but to no avail. The following settings are set

Using the cable it says "Built-in Ethernet is currently active and has an IP address of 192.168.254.1. You are connected to the Internet via built-in Ethernet". It has a green light.

Underneath this against Airport there is an amber light and it says "Airport is connected to the Aple Network xxxxxx. Airport does not have an IP address and cannot connect to the Internet"

In the Airport Admin Utility I have the following

Under the Airport Tab
The Network name appears. 128 bit WEP is set with a password and it is in 802.11b/g compatible mode

Under the Internet tab
Connect using Ethernet
Configure using DHCP
Everything else is blank

Under the Network Tab
Distribute IP addresses is unchecked and everything is greyed out

Don't think the other tabs are relevant since a) I've never used them and b) they appear to be basically blank anyway.

Any suggestions as to what I would have to do to re-establish an Airport -router-internet connection?

Thanks

MalcW