Create a bridge interface for each VLAN you want to associate with one or more SSIDs, call it "vlan100" for clarity (this does not set it to be a VLAN, it's just a name)
"Bridge over" the sub-interface of the physical interface connected to the bridge, eth0.100 for example
Configure the wireless interface to attach to the bridge you just created
Adjust your switch, as needed. Note that many switches have a limit of 16 or 128 table entries, so larget VLAN tags will require use of the vid and pvid options in the config.
(If that's not obvious on how to accomplish, please let us know if you prefer to configure with files, or with LuCI)
As an example of file-based config of the bridge interface:
config interface 'vlan100'
option type 'bridge'
option stp '1'
option ifname 'eth0.100'
option proto 'none'
option auto '1'
option delegate '0'
It is option ifname 'eth0.100' that sets the VLAN tag, not the interface name.
The old post says this is possible through LuCi. Can anyone provide detailed steps?
It appears you are using firmware that is not from the official OpenWrt project.
When using forks/offshoots/vendor-specific builds that are "based on OpenWrt", there may be many differences compared to the official versions (hosted by OpenWrt.org). Some of these customizations may fundamentally change the way that OpenWrt works. You might need help from people with specific/specialized knowledge about the firmware you are using, so it is possible that advice you get here may not be useful.
Ask for help from the maintainer(s) or user community of the specific firmware that you are using.
Provide the source code for the firmware so that users on this forum can understand how your firmware works (OpenWrt forum users are volunteers, so somebody might look at the code if they have time and are interested in your issue).
If you believe that this specific issue is common to generic/official OpenWrt and/or the maintainers of your build have indicated as such, please feel free to clarify.
The syntax used by gl-inet’s implementation is entirely different than official openwrt (it would be invalid on official), so all bets are off with respect to how their fork works. You need to ask them for help - they have a user forum and official support.