Thats a shame, What ISP service name is the DSL ? What brand name and model is on the DSL Modem? Just curious. Never heard of DSL Service not supporting the Bridged mode. So, all it is, It's PPPoE account information that could be setup on the 4500 and there is an RFC bridge mode that needs to be enabled on the DSL modem. This effectively turns off the router on the DSL modem and makes it act just as a modem and passes the data and PPPoE to the 4500, then the 4500 will do the rest. Sounds like the ISP marketing is wanting to market there HW and not help there customers use other HW. My 2 cents.
Yes you can change the IP address if you like on the LAN side of the 4500. The WAN or internet port side of the 4500 gets it IP address from the modems they are connected too. The 192.168 does not have to use used either. I've set mine to my house address, which start at 6.0.#.#. Gets it out of th 192.168 range, possible conficts like your seeing and easier to manage and remember.
If your ISP is unwilling to help you bridge the DSL modem or if in fact the modem can't be with there service then you are probably right to return the 4500 as it's features for you and the dsl modem could be incompatible. Sucks to say. I really like this router for gaming and movies.