Under Setup > Switch Config:
No port was checked for VLAN 0.
Ports CPUPORT 1 2 3 4 were checked for VLAN 1.
Only CPUPORT was checked for VLAN 2.
I added VLAN 40 and I checked CPUPORT and WAN.
Under WAN I checked Tagged.
I clicked Apply Settings.
Under Setup > Networking:
eth0.40 did not appear under Interface Setup > WAN Port Assignment. So I added a line under VLAN Tagging for eth0, set the tag to 40 and priority to 0.
I clicked Apply Settings.
This caused eth0.40 to appear under WAN Port Assignment so I selected it.
It doesn't appear that your screenshot is from the same firmware as the latest firmware available for this router at the time that post was written either...
You're still doing it wrong.
And if you have a CPU port to choose from in the "switch config tab" then it works exactly as shown in the screenshot.
VLAN interfaces are created automatically if the CPU port is tagged.
no other nonsense necessary
there is also no need to set "pppoe_wan_ifname" - all bullshit
we don't need bullshite instructions if you can set everything you need with 3 clicks in the GUI
and eth0.40 will also not appear for selection but VLAN40
You're still doing it wrong.
And if you have a CPU port to choose from in the "switch config tab" then it works exactly as shown in the screenshot.
VLAN interfaces are created automatically if the CPU port is tagged.
no other nonsense necessary
there is also no need to set "pppoe_wan_ifname" - all bullshit
we don't need bullshite instructions if you can set everything you need with 3 clicks in the GUI
and eth0.40 will also not appear for selection but VLAN40