Assuming the TV is no longer under warranty, disconnect the power from the TV and then remove the back cover from the TV.
Locate the cable coming from the control button board on the side of the TV to where it plugs into the TV mainboard.
Gently unplug the cable from the mainboard. Make sure that there are no latches etc that may be securing the connector plug in place and also do not pull out the cable by tugging on the cable wiring. Grab the cable plug and gently remove it from the connector.
Once it is disconnected, reconnect power to the TV and turn it on using the remote control and check if the volume and all the other remote functions now work OK.
If so, you have a faulty key controller control button board - example only
Don’t forget to turn off the TV, disconnect the power and replace the back cover etc after you have finished testing that all is OK with the button board disconnected i.e. don’t leave it open until the part arrives,if you decide to replace the part.
Be safety aware - there is exposed lethal voltage when the cover is removed and the power is connected.