Hello jj77! Clean the contacts, replace the batteries. No go? Disassemble the remote control unit. (Look for hidden screws in the battery compartment.) The buttons ou push are likely a part of a rubbery pad. Clean it completely. There are probably conductive black patches on the rear of the protruding buttons. After cleaning, rub them with a #2 pencil. "Do not color outside the lines." Chean the surface of the printed circuit board they contact. Buff it with a piece of rough paper (not sandpaper - the inside surface of a granola bar box will do) until it shines. Reassemble.
I once bought a used NAD preamp with a remote control. The batteries had corroded and leaked onto the printed circuit board and ruined it. The unit has no knobs or buttons for manual control and is useless. It's a brick. I scrapped it for parts.