@jknott
Where did I do that?
When you suggested to look under Interfaces > Assignments > VLANs.
So, if you want a VLAN on your WAN interface, you have to create it and configure it as needed.
Absolutely agreed, that's what I've been trying to figure out how to do all along.
Solution:
For anyone else who's got this question, just go
Interfaces --> Switches --> VLANs
Edit the entry for the link you want (Click the pencil) <-- That was the part I missed until just now!
Change the VLAN ID from the default of 4090 to whatever matches your external link (eg 180), and click the box to tag the traffic for interface "3". Leave "0" ticked and don't mess with it.
Then you need to go to Interfaces -> Assignments -> VLANs as correctly observed by JKnott, create one in there with a matching number, then back to "Interface Assignments" and choose the new VLAN from the drop-down box for the WAN interface.
Now my pings work, and I'm happy. Hooray!