Have followed the admin guide, and can setup this up fine, however what we're wanting is to turn off any IP connection for the main wan ethernet interface and only have the sub-interface have an IP connection method.
I can't see how you can turn this off, which is causing a double-up of dhcp requests being sent when using both the main/sub ethernet interfaces to use dhcp.
This has been tested on the 521w and 521w-u versions, and I've just used the latest firmware 1.2.4 on the u version.
How can this be configured so that there is only 1 WAN interface that is using IP (the sub interface)
Setting default routes to point to the sub interface doesn't make a difference.
It is not possible to disable the main interface as such (used for all untagged packets).
If you know that all traffic presented to the WAN interface will be tagged and to avoid the second DHCP request, you could just configure the main interface with a dummy static IP address (for example 10.1.1.1/30, if that doesn't interfere with your local addressing).
Ensure that the default route is selected for the sub-interface as you mention above.