Start with the battery — but test it properly
A coin cell that reads fine on a meter can still collapse under the current draw of a transmission. If the fob works when pressed within a foot of the door but not from across a parking lot, suspect the battery even if it 'tests good'.
Rolling-code drift
Keyless entry uses a code that advances every press. Pressing the buttons repeatedly far from the vehicle — in a pocket, in a bag — pushes the fob outside the acceptance window. The car is fine, the fob is fine, and re-syncing restores everything without buying a part.
Water and corrosion
Lake Travis, Barton Springs and washing machines account for a remarkable share of Austin fob failures. Once the board corrodes, a new shell will not help. A quick bench test tells you definitively whether the electronics are alive.
The receiver, not the fob
If neither of your fobs works, the problem is almost certainly the vehicle's receiver or body control module. Buying a third fob will not fix it. This is the single most common wasted purchase we see.