First, try setting one in Forris: https://www.turris.cz/doc/en/howto/guest_wifi and then set the next in Luci accordingly.
Do not forget the “DHCP Server–> Advanced Settings parameter DHCP-Options to set DNS to main gateway” 6, 192.168.2.2 „, but with the correct IP.
I set it up for the first time. You can use the search and your own sense. There’s everything I used in that thread: [SOLVED] AP isolated guest network
The UCI is described here: Https://wiki.openwrt.org/doc/uci