PC behind router can’t see or be seen in workgroup

I have 3 pc and 1 laptop. I joined them all to a workgroup and all can see each other except the PC that’s behind the router can’t see or be seen by the other computers. At first I thought it was just the computer itself, but then I switched computers and it’s the computer that’s behind the router that has the issue. I shut down all firewalls and antivirus programs, but nothing worked. What more can I do?? I have a Linksys wireless router WRT300N version 1.1. The firmware is up to date.