The AUX port is a just a bridge of the MAIN port... so whatever VLAN you assign to main in theory should also be assigned to the AUX. There is no way to assign a different mgmt IP, route or VLAN to the AUX port.
Not sure what you are trying to accomplish, but you could exclude any VLAN/s in the SM's programming. That would allow all VLAN's to pass out the ethernet ports. You could then use a cheap Ubiquiti Edgeswitch 5XP to power the SM plus send one tagged or untagged subnet out one port and another tagged or untagged subnet out another port.