to critters, your link doesn't help becos this router doesn't have the GUI for switch according to thread starter.

I think the first thing to do is to find the port no.
port 0, 1,2,3,4,5. Need to verify which port is CPU port, some switch is port 5, some is port 0, without this info it is not possible to get the tagged and untagged correctly.
Is this switch controller used in other openwrt compatible router, if yes try to find the corresponding port no first. From there only we can start testing on tagged and untagged port.