iPhone volume down button isn't working

Hey guys I bought a used iPhone X and the volume down button isn’t working neither the silent and loud switch but the volume up button is working …. The vibration motor works but sometimes it doesn’t at times ….haptic feedback works on and off , it rings but doesn’t vibrate at times … some times I don’t get any sound at all until. Restart my iPhone I took it to a local repair shop they said it’s the flex under the battery that controls the volume and silent that has my iPhone doing what it does at times is that right ? Or it’s something totally different

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