Surely the way to troubleshoot this is to work out where the requests are getting to.
a) see if any requests are being received to the IIS?
If they are then the inbound router config will be ok. Does IIS try to send out packets?
EIther way it will be useful to look at the router logs for what is rejected and why?
Logically it would be very, very unsual for this to be directly an IIS problem. IIS would have no memory of the packages and where they come from and modify it's behaviour for this.
I always look at these sort of problems as
How could I make IIS/server reject requests from a certain router?
How could I make a router reject/stop requests to a ceratin computer?
It is easier for the second instance. For teh first IIS/server could reject ceratin things based on IP address so if you are *sure* it is not the router then that is only thing I could think of checking.