When you are on the Multiplayer channel on the rFactor2 launcher you could always try searching "DTM" on the searchbar or any keywords that you think may be relevant. Most leagues tend to have their league name or an acronym for it for the server name. Just did a quick search and from what I saw RGPL, HRF1RT, and SIMCO are running a league using the URD DTM mod. The former being a Polish league and the latter two Hungarian. Maybe you could look at those to start. Other than that I am not aware of any other leagues that run them unfortunately. I know Atlantic did a few months back, but participation seemed to dwindle after each race.