X11 forwarding through VPN

I use a linux server through mobaxterm or outty to run Finite element method codes and using vtk application it generates automatic result images. Since the coronavirus outbreak I have been working from home on VPN. Normally I set my $DISPLAY=My-WindowsIPaddress:0.0 and it works great. but through vpn I have a different IP address also a port number that is not 6000. Images are coming out with wrong colors. For eg: all black lines and fonts are coming in yellow. How do set display environment in this case? Thanks in advance! PS: linux server does not have display.