You can probably configured you're linksys to act as the pppoe client, and put in you're username and password and have it simply dial through the modem. Most DSL modems support this, and only act as a router themselves to reduce complexity if you only have a switch or a single computer. This will mean yo're router now terminates the DSL connection, and you shouldn't run into this conflict. To do this, under WAN setup, select pppoe (assuming you're DSL is pppoe based which is the most common I've seen), and enter the username and password from you're service provider.
Another option would be to disable the dhcp completely in the linksys, and just plug the modem into one of the numbered ports (not the WAN port). Thus the linksys will only act as a switch, and the modem will hand out addresses to all clients. This will cause the linksys to not route, but merely switch traffic to the modem interface, which handles getting to the internet. I generally prefer the first option. To do this, find the dhcp settings, and simply disable dhcp.
Last, it is possible to give the linksys an address range on another subnet. You can usually change the device ip to something like 192.168.2.1 and advertise dhcp addresses of something like 192.168.2.50 to 192.168.2.150. To do this, again you need to find the ip address configuration for the management address, as well as the dhcp options.