I could not find any direct instruction for how to set this up so noting here how I got it working.
Here in New Zealand the fibre or cable internet connections (Enable, Chorus, Vodafone etc) sometimes come with VLAN tag 10 from the ONT or cable modem. (I think Untagged or a different tag is used for VoIP)
On the WNDR3800 (Atheros AR7161) the WAN port is on the CPU, it is not part of the switch. So it doesn't show up in the switch config which has an obvious place to set the VLAN id.
( For some other routers the WAN port is on the switch so config instructions are different.)
I can't remember now where I got the hint, but it is actually very easy to set the the VLAN ID of the WAN port.
In the web interface
- Go to Network ▸ Interfaces
- Click the Edit button
- Click the drop down menu for Device: probably currently says "eth1"
- Click in the very bottom field that says "-- custom --" and type "eth1.10"
- Click Save, then Save and Apply
- Repeat for WAN6. You may find that eth1.10 is now in the list of available Devices
Once the changes have been applied, the icon under WAN will change to a little 3 port + cable image.
Hovering the cursor over this will show various details including "Type: Software VLAN"