Please, advise on the right approach

I have the following physical setup:

  1. Open WRT DSRobot router with 2 eth ports is connected to modem on port eth0 and to switch on port eth1
  2. Unify Access Point that runs OpenWRT connected to the switch

I am trying to establish 3 networks:

  1. Home LAN - for family members
  2. IOT LAN - for IOT devices
  3. Guest LAN - for guests

I've already set up 3 SSIDs for each network on the Access Point. I've also created the firewall rules for LANs interactions on the router. I want the router to distribute the IP addresses and keep the AP as a dummy bridge. I know I need to set up VLANs for each network, but I am not exactly sure how and on which devices. Thank you.

So your "Managed Switch" supports VLAN's?

Anyway as both your OpenWRT devices support VLAN's the easiest is to just configure them on that both systems.

So with that you would need to configure 3 VLAN's on the AP and then assign each SSID to one of the 3 interfaces.
You would then configure the same (number) VLAN's on the router and then you can define the firewall rules on the router.
If you want to serve DHCP from the router on each VLAN/SSID you would need to configure 3 different DHCP entries.