Server Connection Issue

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After moving house and connecting with a new router, I have been unable to connect to any online TF2 servers. I can see the server list, but connection to any server fails after 4 retries. The same problem seems to occur in CS:S, Borderlands 2, and Tribes: Ascend as far as I can tell, but not for Killing Floor (I am able to connect), so perhaps it is a steam connectivity issue.

It seems to be a router issue, as I was able to connect on the same computer while on a different network. Another computer on the problematic network has the same issue.

Things that I have tried:

- Deleting the TF2 folder etc. (issue isn't specific to TF2 anyway)
- Forwarding the ports for TF2
- Disabling the router firewall, all my firewalls etc.
- Mucking around with the router settings (it's a Netgear WNR1000v2)
- Restarting everything, a lot

This has been the case for a few weeks now, and any help would be massively appreciated!