Omnirig is a great piece of software that allows to control via CAT many different radios from multiple programs simultaneously. It uses .ini files to define the communication parameters with each different radio, however the Icom IC-9100 description file that comes by default (IC-9100.ini) sets the digital communications modes to RTTY in the rig, what is not correct for most modern digital modes (FT8, FT4, etc.) and in general whenever you use an audio interface between the computer and the rig.
In order to overcome this issue you can download this modified version of the IC-9100.ini description file that maps the digital modes to USB-D or LSB-D in the rig. Just extract the IC-9100.ini file that is inside the downloaded ZIP and copy it to the "Rigs" subfolder of Omnirig in your hard disk, replacing the old file.
Notice that Omninig should have been installed outside the "Program files (x86)" path, because otherwise Windows will make tricky things and could not actually replace the file, even when it looks it has been successfully replaced. The best way to avoid trouble is to install Omnirig in a folder outside the "Program files (x86)" path, for instance in "C:\Omnirig"
If this description file does not work for you and you still get the digital modes set to RTTY in the rig it is simply because Ominrig is still using the old file. Please do no send me inquires regarding this, it is something you will have to fix, I can't help you.