CAT.
At version 2.34.0.0 the FT-991 does not work with HamLib connection and you have to use OmniRig.
Log4OM CAT settings:
OniRig settings:
Yaesu FT-991 settings:
029 232C Rate: 38400 bps
031 Cat Rate: 38400 bps
071 Data PTT Select: RTS
072 Data Port Select: USB
Other settings: I have left as Default.