This is a tough one - ports dont work after a router change with LAN

i have 2 window 7 PC's connected by a router / modem .
I run an application within the LAN only that lets a mouse move between the 2 PC's

when i changed the router this app no longer works.
Windows firewall is off. If disconnected my other firewall, same thing

the porrts the application uses still show open, its TCP listening.

using network monitor i can see the packets come in with right port numbers
but the port just doesnt connect. So its not the router blocking the packets .
The host names didnt change for either PC.

the IP host adresses are different from one modem/router to the next.
Different gateway address, DNS but the packets get routed right it seems.
the new router people tells me they
dont interfere when its traffic between P's in the same LAN.

the subnet mask is the same on both PC's.

internet traffic works fine.

the PC's are in a Homegroup.

any ideas ??

in both cases i cant ping the other PC .even in the case where it works
and i can share files with both modems

all firewalls are down.

same output ping NG in either router
synergy is port 28000

pc1 is 1.0.0.2 pc2 is 1.0.0.3 gateway 1.0.0.1 subnet 255.255.255.0

ive used 1.0.0.2 for years. probably assigned by ISP
i have DHCP