notes that "tcpudp" is a valid value for "proto".
I followed these instructions to create a guest wlan:
The instructions include the command:
uci set firewall.guest_dns.proto="tcpudp"
When I view the firewall "traffic rules" in Luci (OpenWrt 19.07.4 r11208-ce6496d796) it reports that "tcpudp" is an "unrecognized protocol". [Edited to correct typo.]
Not sure if this is a bug in Luci or an error in the documentation.