Remote lock/unlock not working

[FourPointEight-is]

I got 2 keys with my car. One of them looks pretty beat up - obviously that is the one the previous owner was using a lot. The other looks slightly less well used.

The beat up key - it starts the car, locks/unlocks the doors by putting the key into the door lock and turning but none of the buttons on the key work.

The less beat up key - starts the car AND the remote lock/unlock work great.

“Must be a busted battery” I thought. Took apart the key, looks like it’d been open before (bad sign). I got it open, used my multimeter to measure the battery voltage - reads 3.0 (second bad sign). I decided to replace the battery anyway at that point since I had the thing open so I went ahead and desoldered the old battery and soldered the new VL2020 battery on there.

Went out to the car - still no go. Pressing buttons on the key has no reaction. But my other key definitely works.

Is there some sort of pairing that needs to be done to make this work?

Car is a 2005 4.8is