up/down channel buttons
My brother had a Vizio that at first seemed to have the same problem with the up/down channel button, both on the remote and the set. We found that we had not performed the set up correctly and after we did, including the channel search option, the buttons worked correctly. We had to first allow the tv to search all available channels and then the up/down function would work. Even though it's not the same brand you might give it a try.