There is a small subset of DSL routers your can third party firmware on these generally have models from the same vendor that do not use dsl.
Third party firmware does not have the drivers for dsl in it...something about open source licensing. So if you look hard enough you might find a image that would load but the DSL part would be ignored. This is a high risk move because loading firmware that is not meant for the router you have can result in bricking it which many times is painful to resolve and sometime is impossible.
You can use a dsl router as a switch/AP if you like.