VLAN configuration for access with vnc

HI
We have a watchguard firebox x500 without installed and configured by an external company.
It is connected to a nortel baystack 450 switch in the default VLAN1.

Now we have created a second vlan with that has 2 ports (for 2 computers) and they have to be isolated from the network but have to be accessable with vnc over the internet.

I added the firewall port in vlan 1 and 2.
Everything works fine for vlan 1. But in vlan2 the computers can only ping each other and nothing more.

Every port is configured default with untagged access. When I change the firewall port on the switch to tagged trunk, nothing works.

How do I need to configure the ports for the vlan?
How do I need to configure the firewall?