For a school assignment we have to set up a network with VLANs and Wifi but I can't get it to work. 

I have a Netgate firewall, on which different VLANs are defined, an SG2428P is connected to the LAN port on which I also created the vlans. The fixed PCs get a correct IP address according to the VLAN. But I am mainly having problems with everything around WIFI, I have an OC200 and 2 access points EAP245 connected. All devices must be in management VLAN99 according to the assignment. The Netgate is 192.168.99.1, the OC200 is the .2 and my SG2428P is .3. 

Port 18-23 is VLAN99 and port 24 is the trunk to the netgate. 
VLAN40 is the VLAN for the enterprise wifi and VLAN50 is the VLAN for the GuestWifi 

How do I best set everything up? 

1. I can't find the switches to adopt in the OC200 
2. The access points are found and I could adopt them, but the 2nd AP is connected to the other via wifi 
3. I have created 2 SSIDs, these are also displayed on the smartphone but I can't connect. 

Am I doing something wrong with tagged or untagged on the ports where the APs are connected? Am I setting the wrong PVID on those ports? 
Unfortunately I don't see the solution.