Using RealVNC with Windows SBS 2003

I’m trying to configure my SonicWall TZ180 router and RealVNC so I can access a specific Windows XP Pro workstation on a network running under Windows Small Business Server 2003 DHCP. I forward port 5900 as required by RealVNC to the workstation PC in the router. However, i’m not able to connect from home to the workstation. I have a static IP address from the service provider. I think this issue is due to the fact that my Windows server is setup as DHCP controller, not the SonicWall router. Please help!