Help: People are unable to connect to my TF2 server (router issues)?

To start off with, I have a ZyXEL ZyAIR G-2000 router and I have the following ZyNOS firmware version: V3.60(HI.1)

Whenever I am connected to the internet via my router and set up a dedicated server on my PC for Team Fortress II (using the Team Fortress II Dedicated Server program that is provided by Steam), my server does not show up on the Internet Servers list, and people are also not able to connect to my server by either manually adding my server’s IP to their favorites list or by joining through Steam; whenever my friend would try to connect, he would get a “Server does not respond” message. Note that I am able to join other servers just fine when I am connected to the internet via my router; it is only that nobody can join my server. On the other hand, when my PC is directly connected to my modem and my router is not involved at all, there are no problems: my server shows up on the Internet Servers list, and my friend (the same one who cannot connect when my router is involved) is able to connect to my server.

I can’t see any reason why my server shouldn’t show up on the Internet Servers list and why people can’t to my server when my router is involved, as the firewalls on both my PC and my router are off, services blocking on my router is disabled, and all ports (both for Steam Client and Steam Server) that are needed for this game (which I obtained from portforward.com and then cross-referenced with the information on Steam’s website, so I know that I am not missing any ports) are forwarded to my machine on our LAN; I setup a local static IP address on this machine so that my local IP address is always the same for port-forwarding purposes. Additionally, the default server is set to 0.0.0.0 on my router’s SUA/NAT page, and all of the local IP addresses on our LAN are mapped to one single global IP address (the global IP address is not static) from our internet provider (many-to-one address mapping scheme).

What makes this issue even more interesting is that I am able to host a dedicated server for at least one game (it’s possible that I may be able to host servers for other games as well, but I haven’t ever tried to host a server for any of the other games that I play as I have no need to do so) on my PC when I am connected to the internet via my router: I have been hosting a dedicated server on my PC for the game Star Wars: Battlefront II for over three years with no issues of users not being able to connect to my server. I have also never needed to forward any ports for this game.

I don’t know if this may have any relation to this particular problem, but I also see these access blocked notices in my router’s logs quite frequently:

Packet without a NAT table entry blocked: ICMP(type:3, code:3)

Packet without a NAT table entry blocked: ICMP(type:3, code:11)

Any help to resolve this issue would be greatly appreciated.