Using a TOSLINK cable, connect the ADAT in of your UMC1820 to the ADAT out of the additional device, and the ADAT out of the additional device to the ADAT in of the UMC1820.
Connect the UMC1820 to your computer via USB 2.0 cable, making sure to first have successfully installed the most recent drivers for the unit from its product downloads page.
https://www.behringer.com/Categories/ Behringer/Computer-Audio/Interfaces/UMC1820/p/P0B2J/Downloads#googtrans(en|en)
Open the UMC Control Panel – you should now see the additional ADAT inputs and outputs. In order to use these for recording, simply open a DAW which supports ASIO and select this as your audio device driver, and you should see the ADAT inputs alongside the device’s local inputs.