I wanted to start over, so I tried resetting the configuration. Ethernet never showed activity, so I tried the other reset modes and they all act the same way.

1. The device beeps and all LEDs except the Ethernet come on for half a second then off.
2. Like ten seconds later the same LEDs flash and beep twice
3. At this point the power light is on (no other LEDs) and the connected laptop shows “Cable unplugged”.

How do I get back to communicating with it without taking the board out?

Model: RB912UAG-5HPnD-OUT-US

beep twice usually means it thinks it is ready to start running.

You could try connecting to it via wifi, it may have some mikrotik-XXXX ssid with no password.

If it is similar to a map/wap, then Wifi is often the only way to connect to this type of device when factory defaulted, but usually
the ethernet port is active, just marked as WAN and firewalled.

Alternatively you might be able to connect a serial adapter into its usb port and get a console session that way.
The mikrotik woobm was good for this.

Otherwise doing a full netinstall procedure will perhaps be required.

I ended up returning it as I couldn't get any communication with the device (even when trying netinstall).

Thanks for the help though.