The peculiarities of FT-891 are described in this post.
The bottom line is that you’ll either have Digirig connected alone or Digirig and also a direct USB cable from transceiver to the USB. In former case you’ll see the COM port from Digirig, in latter you’ll see Digirig’s COM port and also radio’s own COM port. Use Digirig’s COM port number for PTT by RTS, if you have CAT connection then use radio’s COM port for CAT control. If direct cable to radio is not connected then disable everything in CAT section of the software configuration (rig=NONE).