wifi guest vlan

Hi, this is a long/detailed question with the hope to minimize follow-up questions. I’m about to get crazy with this issue and I would really appreciate help. My setup is: ISP -> router running OpenWRT (OpenWrt 21.02.5) -> Trunk (vlan 1, 3t, 4t) -> RT-AC68U (Merlin FW 386.9, AP mode) -> WiFi...