I tried to figure out which model RCA universal remote you have based on the keys you mentioned. The result I found was the RCRN04GBE. It might not be the model you have, but the findings might apply anyway.
The manual for the remote states that it has a “Channel Lock” feature that always ties the program up/down buttons and number pad to the TV. You might need to turn this off. It also has a “punch-through” feature which can make the volume buttons control a different device, but this is done automagically by the remote based on the devices it thinks you have set up.
If disabling the Channel Lock feature doesn’t work, you can reassign the SB·AUDIO mode to control a different type of device. This might unlock the number pad. I would recommend setting it to control a TV since that should make the most buttons work. Then you just need to pick any brand of TV that uses the NEC protocol (most Japanese, and also most Korean and Chinese brands do I think) and set up the remote to control that brand.
This should give a sufficient number of commands to which the OSSC can respond.
Link to the remote manual: