TF2 Can't connect to same server as someone on same network; Connection failed after 4 retries

[kerneldeimos]

Hello! I'm having a problem, which has been asked about many times, but throughout my Googling I haven't found any helpful (for me) answers. I can't connect to any of the servers in TF2, because I get an error reading "connection failed after 4 retries". I can host and join LAN games just fine.

EDIT: I can't connect to any servers if and only if my dad is on the server I'm trying to connect to. I figured it was all connections since we tried connecting at the same time. This problem also occurs in the game Quake Live. Is it because I'm on the router's DMZ?

Details:
- I'm on the router's DMZ, therefore all ports are open
- I'm running no firewall (which is a really bad idea, but I digress)
- I'm running Ubuntu Linux 14.04, with no antivirus if you hadn't guessed
- I'm behind a router & modem. Other people in the house connect fine.
- I'm on WiFi. Others are wired. Switching to wired isn't really an option.
- The list of servers appears without issues. Some people had a similar issue where no servers came up.

Things I've tried:
- connecting via the terminal
- opening backpack screen (read it might help) and connecting via the terminal
- assuming fetal position and crying. a lot.

[kerneldeimos]

I solved the problem! Disabling UPnP - for whatever reason - enables 2 people with the same IP to join a server. I'm assuming this has something to do with the way the game is programmed though, since some other games (Minecraft, for instance) don't have this problem.

I'm still not sure why this works though. Does anybody happen to have a good explanation?

[The Protector]

UPnP???
What is it???, help me please :'(

[kerneldeimos]

Uh, I don't remember completely (posted this in 2014), but I'm pretty sure it's a router setting. If you go to your router's ip in a browser (usually 192.168.0.1 or 10.0.0.1) you'll should find the option somewhere (probably under an "advanced settings" tab or something similar)

[The Protector]

Ah yes, thank you, but unfortunately, it does not work, thank you for trying to help me. ^^

[КрАсОтКа]

vac

[The Protector]

?