You can absolutely use the UDP-205 as a streamer. I've done this with my UDP-203 in the past. What you need to do is connect the Oppo to your network using either wired Ethernet or Wi-Fi. Go to the setup menu and test the internet connection (it may take a while for the Oppo to obtain an IP address from your router).
Once the test is successful, the Oppo will advertise itself on your network as a UPnP renderer. Cambridge Audio has a good explanation of UPnP (link). Now that the UDP-205 is a renderer, you can stream to it using an app installed on your mobile device. I use BubbleUPnP on my Android phone or mConnect HD on my iPad. You will need to log in to Tidal on the UPnP app and select the Oppo as your rendering device (tap 'Play to' on mConnect or the cast icon in lower right in BubbleUPnP). Then just search for some music and hit play.
If your music is stored on a PC using Windows, you can enable the built-in UPnP sever so that you can use BubbleUPnP or mConnect to browse your music files stored in your Music folder and stream those to the Oppo over your local network (link). If music files are stored on a Mac, you can download an app called Kodi that can turn the Mac into a UPnP server (link).