The volume control isn't through the cable box but through the tv. Have you tried a DVD player or something else attached to the tv to see if the problem replicates itself. Have you tried to use the actual TV remote? Try attaching that box to a different TV?
Mine is doing the same thing, worked fine for a few days and out of the blue it won’t control tv or sound bar. I have Samsun 6 series, did you find a fix?
Go through the re-pair procedure.
Is the cable-box connected to "HDMI 1" on the Samsung tv hub? Go though the Samsung TV Set up (found on menus) again.
Do you have additional speakers attached? If yes, the switch to television speakers. Are they working? If yes, then switch back to other speakers. They should work if the box is turned on.