Nice find, you just stopped me from returning my Flint due to the inability to configure VLAN in Luci. I tried your suggestion and was able to segregate my home network into different VLANs. Thank you so much for sharing this.
Got my flint today and hit the enable VLAN functionality checkbox in luci. Couldn’t add VLANs afterwards. I’m not alone Because I need VLAN7 on PPPoE WAN I just entered the custom ifname eth0.7 for WAN and it works. My Creta uses GL.iNet’s OpenWRT 19.07.8 firmware but the Flint’s firmware isn’t polished and incomplete (OpenWrt Chaos Calmer 15.05.1 a828782+r49254 / LuCI Master (git-20.353.27956-6404588)). I also have devices from other vendors with vanilla Openwrt 21.02.1 and luci material theme. This qsdk really needs to be bumped to the latest release.