The dspMixFx app has no influence on Cubase. I am fairly certain your problem was caused because the ASIO driver was not selected in Cubase which could have been caused by the UR28M not being turned on when Cubase was started or being turned off and back on while Cubase was running. This is important other wise if the unit is turned on after Cubase starts, you will have to re-select it in the control panel (and possibly need to remap it in vst connections) but the hardware racks may not be visible in the mixer.
To ensure you’re set up correctly, turn the UR28M on prior to starting up Cubase.
In Cubase Device Setup go to VST Audio system and select the Yamaha Steinberg USB ASIO driver in the ASIO Driver pull down.
There will now be further choice for the Yamaha Steinberg USB ASIO under VST Audio System in the left pane. Click on this and you will be presented with all the ins and outs of the unit (most should be Active except possibly the S/PDIF ins and DAW outs). At the top of the right panel you will see Control Panel in which you can set your buffer size and sample rate.
After closing the Device Setup window, select Audio Harware setup from the Devices menu and you will see the window shown in the documentation in which you can make various HW adjustments.
Next step would be to open the VST Connections window and select or control the inputs and outputs.
Finally, in the mixer, ensure that racks are selected in the layout, that input channels are selected in channel types, and then hardware selected in Racks.
At the bottom of the hardware graphic in the rack you will see a headphone symbol and a bargraph. Clicking on either of these also opens the UR28M Hardware setup with the headphone select tab or reverb tab selected respectively.
Hope this helps.