Oh wow, apparently I didn’t see the “DSL” in the model name. You could try to figure out whether it is physical identical to another model that doesn’t have a modem built-in (except for the modem obviously) and use an image for that, if you don’t need the modem functionality.

Oh, you’re right. In this case: https://freshtomato.org/

https://www.asuswrt-merlin.net/ as a first step.