Hi xize11,
To setup vlan on flint, the luci - Network - Switch page should not be used.
If you want to tag which port, just add ethx.y(y is the tag) to firewall zone.
For example, add lan port 4 to guest zone(pre-configured by gl script)

uci set network.lan.ifname='eth1 eth2 eth3'
uci set network.guest.ifname='eth4'
uci commit network
/etc/init.d/network restart

if want a tag.

uci set network.guest.ifname='eth4.20'

Hope I understand your requirement correctly and above may help.