Cannot setup NAT on edimax router

Hi all, I ve got this setup:

It works well - I can connect from TESTPC and LAPTOP (bottom left corner) to SERVERS A/B/C (bottom right corner) without any problem. Everything seems to be routed perfectly, however from SERVERS A/B/C I cannot ping TESTPC and LAPTOP, and I can't even establish an RDP connection.

I was told to setup static routes or NAT, but I am at a loss on how to do it. I am using an EDIMAX BR-6424NL wireless router - can you help me out??