(Sorry, I was in a rush when responding above....)

Access Point mode is a complicated situation because all of the devices are in one LAN IP subnet. When communicating with other devices, the first thing that happens is the destination IP address is compared with the IP subnet. If the destination is in the same IP subnet as the host, the host does an Address Resolution Protocol (ARP) broadcast query, "who has this IP?" Some device answers saying, "IP xx.xx.xx.xx belongs to MAC address xxxx"

If the destination address is not in the same IP subnet, the host sends the packet to the Gateway IP address (using the same mechanism).

Every device (router, satellites, that Ubiquiti Dream Machine (UDM Pro) contains Ethernet switch modules (the Netgear MS510TX is be definition an Ethernet switch). These modules remember every hardware MAC address that can be reached through each port on the switch, even if the data packet has to travel through several switches to reach the final destination.

For example, the UDM may remember, "I saw a packet from MAC address A that came into me on my Port 3. So, if I want to communicate with A, then I transmit out Port 3." The switch has no idea where that device is, only that is "out Port 3".

If Port 3 is connected to the MS510TX port 7, the MS510TX may remember, "MAC Address A can be found out my port 2".

This process continues until eventually that packet is delivered to MAC A. Each Ethernet switch has a table of how many MAC addresses it can remember. The MS510TX, for example, holds up to 16,000 MAC addresses.

When the Orbi router is in AP mode, all these ARP broadcasts and the responses flow through it. A device connected to the router can then be reached from anywhere on the LAN. Likewise, devices connected to the router can reach anywhere on the LAN.

The problem is that the router "knows" that satellites can be found only through the router LAN ports. If satellites are connected to the router LAN ports, then the LAN Ethernet switch says, "Sure. That satellite MAC is connected to Port (whatever). Out you go. Have a nice day." The packet may go through dozens of switches, but eventually it will get to the satellite.

If satellites are NOT connected to the router LAN Ethernet switch, it says, "you must be kidding. I have asked over and over (ARP), "who has IP xx.xx.xx.xx? and nobody responds!" (They cannot respond because they are not connected to the LAN port.)

Could Netgear have written the firmware to look for satellites on the WAN port? Sure they could. But they didn't.

So, everything can connect, but the router cannot find the satellites and report anything about what is connected to them. The Attached Devices display and the Orbi 'app' are useless.

The way to "solve this" is to connect the satellites to the router LAN ports.

By physically moving the Ethernet cable from the switch to the Orbi LAN port, or
If the switch and router are in different locations, by creating a VLAN which can be used to get the traffic where it needs to go.

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R700P