Have you tried replacing the battery in the fob?
They generally take typical coin-shaped batteries.

Can't even figure out how to open the fob. Not sure it's possible. Even OCD DH can't figure it out, and boy did he try. AI says this, though: Older Models: Use a small flathead screwdriver or a thin object to pry open the case at the designated notch along the seam. Guess we can try that. Not such a big deal to use the key with the Sienna, which has a key lock on both front doors, but the 2013 Camry only has a keylock on the driver's door. Clearly it has not been a pressing issue for us. The one thing I have missed is the ability to make the car make noise at the click of a button, when we don't remember exactly where it is in the parking lot, and it is nice to pop the trunk when approaching the car with groceries in hand, but yeah, not a pressing issue.

IP