Firewall/NAT

Hello everybody! 

I have a mysterious problem with the NAT-forwarding on my 1.33 Monowall.
I have a Monowall behind my landlords router. He has forwarded port 5000 to 6000 to my Monowall (static IP), and I have my own subnet inside.
When I try to NAT port 5000 to my server on port 22 (SSH), I can't get through. But if I NAT port 6000, I can SSH in to my server using port 5000.

I have no idea how to solve this problem.. I have tested the forwarding from my landlords router using my laptop, and that works just fine.

So forwarding port 6000 opens up port 5000... But forwarding port 5000 only doesn't help me at all.

Any ideas? 

Could it be that your landlord hasn't actually forwarded the range 5000-6000 to your m0n0wall, but instead forwarded external port 5000 to internal port 6000 (on your m0n0wall's WAN IP)?

No, I checked the forwarding my self, twice (I configured his router as he doesn't know how to).. anyways, I tested it on my laptop with success, so the forwarding is correct..

Very, very strange...