@URL I'd double check your VLAN and SSID set up to make sure you have configured the VLANs correctly. Is that the only SSID/VLAN the issue occurs on? 

With regards to the red links, double check your connections or the device logs; it may be reporting some info that would be helpful in finding the problem. Normally I advise to check the physical cables first; are they testing correct? 

As for your last question, your network seems pretty standard; I recommend the official FAQ page, not just the user guides:
Documents | Omada Network Support

  @URL 

This looks to me like you have trunking issues - somewhere you may be flipping a tagged network into an untagged, and/or your SSIDs are not set with the correct vlan tags

Its best practice to have only one untagged vlan on a trunk, and the rest tagged.  Generally this is management [untagged all the rest [tagged] and keep it consistent all along the chains from gateway to end devices

Access points simply insert the configured tag for the specified VLAN into the traffic for the clients - they dont actually move them onto other networks, so if, between the EAP and the Gateway or DHCP server that tag vlan gets muddled up or placed on an untagged trunk you can get weird stuff like this happen