Just purchased and set up this Q70T TV 65". Remote does not change channels with COX mini box unless it is less that a foot away from IR. All other functions appear to work normally with an AppleTV 4k box and 4k BluRay player (Samsung).
You can try replacing the batteries in the remote but this issue isn't really related to the Samsung Remote since it is going to send the same strength of IR signal, regardless of device.
I would like to add that new alkaline batteries were installed and when programming the mini box, I got the channels to change during the setup. After the fact, it didn't work.