try to look at this - Omnia Switching Vlans and interfaces
you need to set WAN port to trunk and configure at least VLAN1 as native (untagged I assume) and VLAN6 as tagged. then set VLAN6 as access on eth port you like. And assign rest of ports including wlan to VLAN1 (I’m not sure how openwrt works if you start using VLANs - if you had to use them everywhere or it will use VLAN1 as default if you don’t specify it)
you probably don’t need to configure access/trunk mode and it will be done “automatically” by setting tagged/untagged/off. The example you’ve linked is quite nice and explaining. If I understand it right you nearly got it working. You probably “just” need to set VLAN6 as tagged on WAN port. (I’m not sure if you need to set VLAN6 in CPU )