Datto access points communicate with Datto Networking over their default untagged network connection. The port the access point is plugged into must be set up to handle this untagged traffic. Most managed switches can be set up to tag outgoing untagged traffic onto a specific VLAN, and untag incoming tagged traffic from a specific VLAN. The VLAN this traffic eventually ends up on must have a working DHCP server and a route out to the internet.
Look for PVID or "native VLAN" settings in your switch's configuration settings. Refer to your switch's manual and support documentation for full details on setting up ports and VLANs properly.