Key Not Detected

Came here because I had the following issue:
1st Key fob - works fine.
2nd key fob - was dead for months, recently replaced the battery, but does not open my 2015 Prius.

The 2nd key fob flashes the tiny red light when in proximity to the car, or when pressing any of the buttons.

No aftermarket modifications nor electronics plugged into the ports. Just simply won’t connect.

I pulled out the battery and touched it to get power button, but that had no effect. The physical key unlocks the lock when I turn it in the door.