UAP-AC-PRO (GEN2) Multiwan & Load Balancing

My UAP AC PRO (GEN2) is already OpenWRT. i have 3 uap ac pro and its already openwrt.
my first uap is using WISP/Repeater mode. i already install mwan3 Load balancing / SQM, but i cant make it work or it's not working or because it's not a router with a dedicated wan port.

i want 1st uap to make one of Lan as Wan make a Load balancing.
and the other 2nd uap is just a wisp mode with lan cable to the 1st uap.

youtube toturial helps me but they are using router.

im new in here and using openwrt.