The Ethernet ports on the access points can operate as either network uplink port used to talk to the local network and internet, or as a wired client access port used to pass a wired client's traffic onto the local network.
The access point will listen for DHCP traffic on each Ethernet port, if it hears an active DHCP server responding to lease requests, then the port is set as an network uplink port. If it hears DHCP lease requests, but no DHCP server responses, then the port is set for wired client access. This is an automatic process, and cannot be manually configured.
When the port is set for wired client access, it will treat the client as if it was connected to one of the configured SSIDs. The SSID used is set under "Configure -> Advanced -> Bridge wired clients".
Since wired client access mode requires the client device request an IP address via DHCP, devices that use a static IP address will not work.
If you would like the client device to be bridged directly onto the local network, ensure "Bridge wired clients" and "Bridge SSID" are set to the same SSID.