Using FT2000 as my radio selection, I was finally able to have RMS Express set the frequency on my FTdx3000 when I made my Channel Selection, but no TX occurred until I selected FT-991/A as the PTT Port (optional).  Also selected USB as opposed to USB Digital under the Radio Setup.

I'm also using LDG's YT-1200 tuner that communicates with the radio via RS-232 while the Yaesu-supplied USB cable communicates between radio and Windows 10 PC.  So, a bit of a hassle because I have to select a Channel based on propagation data while in USB connect mode, switch to RS-232 mode (Menu 037), let the YT-1200 do its work, then switch Menu 037 back to USB mode to complete the Winmor communication.  A bit of a hassle, but it works.

I had some problems with T/R latency initially, but found that turning the Carrier down to 15W, cured that little problem, and communication was finally established and messages sent.

What setting are you using for CAT, Menu Item 037, RS232 or USB?

What baud rate?