You need to setup your audio interface in Cubase.
Meaning, you need to setup and assign each of the UR44 input and output connections as input and output busses in Cubase. Then each track in a project needs to have a input buss assigned to it. The default stereo output will automatically be assigned to all tracks in the project unless you change it (don’t).
Setting up the audio interface is done in the devices>device setup>vst audio system. Choose the AISO driver for your UR44 there. Then go to devices>vst connections and set up your UR44 I/Os on the input and output tabs. After that you need to assign an input buss to each track. One place to do that is in the inspector section.
Typically the front input on the UR44 where you connect the guitar is set up as a mono input buss which will be assigned to the audio track you intend to record the guitar too.
All of this info can be found in the Cubase operation manual and many youtube vids are available showing how to set up your audio interface. You really should review these a bit too. Also, please list your system and hardware specs for further assist.