This might work...I had a similar netgear wireless router which I tried forever to get to work with another modem, and wouldn't until I tried some a little bit indirect.
- Turn off the DHCP on router, and puts put its ip as 192.168.1.101 or whatever, same subnet as modem, but not ip.
- then goto basic settings i think it is, say you dont need a password to login. the put in the ip details. here is the crazy stuff, but it worked for me, but not the exact same situation. tick the box to chose static ip, not assigned from isp. put as the gateway the ip of the modem, so 192.168.1.100, give yourself any ip in the same subnet, but not the main ip you set in lan settings when you turned off the dhcp. 255.255.255.0 as the mask. and i think that's it. it may not like it as the adsl modem is still going, or it may work, worth a try.
- if it works, you will have to set ips of all the other computers manually as you have turned off the dhcp.
could be bullshit, i did this ages ago, and its always easier when you're sat there in front of it fiddling with the setting for a day!