Hello! @Leithiαn
I'm sorry for this error you have. Have you tried to restart your router? In case you did not, keep it off for at least 30 seconds, then turn it on again. This solves many problems. Also try to connect with an internet cable.
If this does not work for you, check if steam requires updating, and check the integrity of the ETS2 files through steam (right click on ETS2 game, properties, local files and verify integrity of game files.) Make sure your PC is updated, drivers, etc.
You also try this https://documentation.cpanel.net/display/CKB/How+To+Clear+Your+DNS+Cache and clear your PC temp files with CCleaner. You may also try deleting and re-downloading your TruckersMP.
Let's see if there is someone who can provide more solutions. Keep us informed.
Regards!