TREADINATOR

This is the SIMPLE way of keeping your copy of ETS2/ATS compatible with the TruckersMP addon - there's no fiddling around with folders or backups here, and it's literally 2-3 clicks! (Provided the games haven't updated without you noticing)

Assuming you have Steam set so that it doesn't open automatically when Windows starts:
Open Steam. You'll notice a Downloading section at the bottom - these are the updates pushed from SCS. Click there, then click Pause on the ATS/ETS2 updates. That's 2-3 clicks. That's all there is to it. Your game remains compatible with the TMP addon

When TMP updates, you may resume ATS/ETS2 updates

If your game has updated already and you can't play MP, just wait for TMP to release their update. It's usually only 1-7 days depending on update content. Patience is a virtue, my friends