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Quoted from Loe

Try out what happens when done the other way around ( your brother plays and you join a server after that).

I connect to the server and begin to time out as soon as I get on.

Quoted from fuo

Start up TF2 with -clientport 27016 in the launch options one on of the computers (not both)

Tried it on my PC only, his PC only and then one with port 27016 and one with 27017 and none worked.

Quoted from Waebi

if that doesn’t try changing yourself and him from DMZ to auto-assigned (DHPC) and see if that changes anything.

Done – no difference, I was in DMZ just to get around port forwarding for games where I needed to host (Worms, Borderlands etc)