First, make sure that you’ve installed the latest UMC Driver from the UMC1820 product page. The driver is installed in three parts: ASIO, WDM (Windows Driver Model, which incorporates MME, DirectSound and WASAPI), and finally MIDI.
ADAT channels are only available when using the ASIO driver. Therefore, in order to make use of the ADAT channels, you need to be using software which supports ASIO. For this reason, you may find that the ADAT inputs and outputs do not appear when you look at your Audio Devices in Windows, but will appear when you use a DAW which supports ASIO. When checking your DAW audio device settings, ensure the ASIO driver is selected. For example, in Reaper this can be found in Options > Preferences > Audio > Device.