OK after struggling with it for a (big) while i have found the solution, by reading this post
The problem was not on NethServer but on… Windows Server with 2 NIC.
NIC#1 is connectect to my main network with gateway pointed to main router.
NIC#2 is connected to my second network but it has not setup gateway to second router. On debian based linux port forwarding was working in this case, but on NethServer it is not.
So I have to input to NIC#2 gateway IP of secondary router (of course WinServer shout on me because this should’nt be setup like this…)
I found it by setting up also OpenVPN and when I was unable to get into the second LAN, I have started to search the forum for a solution.
I am curious why NS need to be set as GW and debian based distro handle it without problems 