You might first need to turn on the Voice Assistant setting and then choose Bixby, Alexa or Google Assistant. I've found that Alexa and Google Assistant work best.

Answering my own question: I am now able to use the One Remote for all of the cable box's functions except future recording and the future TV guide. The One Remote's up and down buttons will not take me to the cable box's A, B, and C buttons on the on-screen display, which control those future functions. To get to other functions, I followed the advice of Cristian, the Samsung ambassador. I made sure my cable box was connected via HDMI and that the Anynet+ setting was turned on (go to Home screen-Settings-External Device Manager-Anynet+). I went to the cable box icon on my home screen, then used the up button to go to the top level, which shows the box's functions. I went to Recordings to see the screen that shows what has been recorded. I also could go to Record to start recording instantly. My cable box is an Atlas URC1056B04, rented through WOW/Knology. Other cable boxes might be even more compatibility with the One Remote.

Thank you, Cristian, for your patience in answering all my questions. I had already done the HDMI connection and Anynet+ setting that Cristian recommended, So I was able to do most, but not all, of the cable box's functions with the TV's One Remote. I certainly can live with that, since the One Remote does everything else I need, I found that the One Remote's up and down buttons will match the cable box's buttons that show what has been recorded. They also will match the buttons that show an on-screen TV guide. But the One Remote's buttons won't let me immediately record what is currently showing. They also will not take me to the cable box's A, B, and C functions. Those functions take me to a future-showings TV guide and to future recordings scheduled. They also let me organize or cancel recordings. Others might find that their cable boxes are even more compatible with the One Remote than mine. To check it out, do the connection and settings that Cristian recommends and go to the cable-box icon on the home screen. Then go up (with the up button) to Recordings.