##UPD:
A funny situation came out = D
Recently reinstalled Windows, respectively, the folder (backup) of programs and other things I saved. And it turned out that TrikersMP took the game's sour from the old backup folder (where there is certainly no DLC), and not from the current one.
Thank you all for your answers. The topic is closed.
Unfortunately, everything is still the same. In TruckersMP DLC does not work.
Thanks for answers.