Proper way to create a trunk in Tomato

I'm at a loss. All I need to do is create a trunk so that the VLAN is accessible on another managed switch, but everything I try seems to not work.

I'm running the latest shibby version 1.38 AIO on ASUS RT-AC66U. Setup is below.



I'm struggling to get a trunk port to actually work. I manually changed the nvram to set trunk_vlan_so to 1 from 0, but still no change. Vlan3 is currently not assigned to any ports since it's used for wifi only, but...

Proper way to create a trunk in Tomato