Remote desktop on iOS/iPadOS refuses to connect over an ethernet adapter if Wi-Fi (and presumably cellular on models that support it, though I can’t test that) is disabled. This is despite being able to reach the defined server using an SSH client such as Blink or an alternate RDP client such as Jump Desktop.
Steps to reproduce on a Wi-Fi only iPad:
1. Set up a working RDP connection in the Remote Desktop app on iPadOS, as well as in an alternate RDP client like Jump Desktop.
2. Confirm that the connection works over Wi-Fi in both clients.
3. Plug in a USB C ethernet adapter and turn off Wi-Fi in the Settings app.
4. Attempt to connect to the configured server with Remote Desktop. Note that Remote Desktop will immediately return an error indicating that no network is available.
5. Attempt to connect to the configured server with Jump Cloud. Note that Jump Cloud will connect to the server without difficulty.
Note that the above steps will also work when the configured server is *only* reachable over the USB C ethernet adapter; it appears that Remote Desktop is not properly detecting network access over wired ethernet, even though it works fine with such access. (Another way that this can be tested: Configure a server that is only reachable over USB C ethernet. Connect while Wi-Fi is on. Disable Wi-Fi and note that the connection remains up and usable. Disconnect and then try to reconnect; note that Remote Desktop will now erroneously report that the network is unavailable.)
iPad Pro 12.9” 4th Generation (Wi-Fi only) running iPadOS 15.6