All batteries are designed to fail eventually and the only remedy is to replace them with new ones.
The most common reason your Fire TV remote isn’t working is a dead battery. If you encounter this problem, you should first check and replace the batteries in your remote control.
We recommend using alkaline batteries for devices such as the Fire TV remote, which work more effectively. Rechargeable batteries are also available.
Source:https://www.waybinary.com/firestick-remote-not-working/