Can't connect to a Mac on LAN trough VPN

I have a ZyWall 110 on which I've configured VPN.

On the LAN I have a Buffalo NAS running and a Mac on which I'm running FileMaker server (FMPS).

I'm using VPN Tracker 365 to access the LAN from the outside.

When I'm on the LAN (on the inside network) I have no problem in reaching the Mac running FMPS or the NAS.

But when I connect trough the VPN I can only connect to the NAS on the LAN.

When connected trough the VPN I can ping the NAS and the ZyWall on it's local ip-number on the LAN but I can't reach the FMPS Mac.

The FMPS is up running and as the FileMaker Server we reach trough our FileMaker client on the WAN as the NAT protocol we have added to the Firewall.

But I also need to reach the Mac running FMPS to send files, VNC etc.

The Mac running the FMPS has 10.11.4 running.

Earlier we had a D-Lind 800 FireWall and we had no problems then. At that time the FMPS did run 10.8.

Best regards, Johan