The issue is with the router at the server's end, where port 5900 needs to be
forwarded to the server's IP.

If you do this, I'd very strongly advise you to set an IP restriction too,
so that access is from your Internet-connection only, as VNC passwords aren't
all that secure. This of course presupposes you have a fixed IP address on
your home connection. If not, then use of a tunnelling protocol such as SSH
or Zebedee might be advised for better security. (although these are more
complex to set-up)