Regarding the VLAN of the Ethernet port (including WAN and LAN) does not support at the present.
However, there is no need to be disappointed. We are currently collecting requirements, and if there is a lot of feedback about it, will also consider developing this feature. Thanks.
Just try it. Add/modify the VLAN from WAN of LuCI probably break WAN configurations of GUI. But that is no problem, just reset the firmware if occur something wrong.