First time VLAN setup - Need Assistance Please.

I have a client that has an existing Mikrotik RB2011UiAS-2HnD-IN Router in place and working.

They have a fiber connection to another building where they want to install an RB912UAG-2HPnD-OUT into a large gymansium.

What I am trying to accomplish is on the AP, have a guest wireless. I can configure the gust wireless network and get it to work however can not get it to communicate back to the main router. I have tried doing a NAT Masquerade on the router and AP.

I also tried setting up a VLAN on the router and the same VLAN on the guest network and no luck.

Just not sure where to start.