[2021] Multiplayer DESYNC

He may have some Steam ports blocked:

Required Ports for Steam - Steam Support

Article "Required Ports for Steam" support.steampowered.com
The best way to test that is for all players to switch to the crossplay game version in the Steam Betas tab for this game. That uses Paradox MP services instead of Steam's.  

Then it is not ports but some other local problem there. Hard to guess what it might be, to be honest!

Is there more than one machine at that location, if so can he test this from another one? This is to see if it might be an issue with his machine, or elsewhere in his network or ISP.

Also is he connected via wired or wifi? Whichever one can he try the other?  

Rosted_the_gamer said: We opened all the steam ports, this did solve the issue in someway.

I don't understand why that helped as using a VPN bypasses all that anyway. And the crossplatform version uses completely different ports than Steam does.

I suspect there is something else happening here ! What upload/download speeds do you all have?  

Not really, sorry, it's a case of trial and error.

The person who has this problem, does he have access to a second machine in the same network? If so does it also see this issue?

Otherwise what if he moves his main machine to another network (if possible!)?

This is to determine if the issue is in his machine or his network.