As Cubase takes complete control over the UR28M, the signal routing of the UR28M in standalone operation (no Cubase in this context) can be completely different. So what you’re experiencing is normal.
There are a number of factors involved here.
First and foremost you need to have your Ins and Outs setup properly in VST connections in Cubase - ensure you have Ins 3 and 4 setup as a stereo input pair. You should see meter activity on the input channels. (Enable hardware in your mixconsole channelstrip and bypass the insert if need be.)
Create an Audio track routed from 3/4, and send this to the outs corresponding to your monitors.
Enable the monitor (speaker icon) for the track.
You should now see activity on the track and output channel, and of course hear it in your monitors.