Installing a "key cylinder" to open and close the garage door
Assuming your opener has the terminals for doing so, to hardwire this you would want either a double-momentary keyswitch, or a housing that takes a lock cylinder and uses its tailpiece to actuate a pair of momentary switches, plus appropriate mounting hardware and wires. The latter gives you more flexibility in picking a cylinder that matches, e.g., your house keys.

I agree that if the battery is good, and is appropriate for that weather, and there isn't an unreasonable amount of radio frequency interference (RFI) with the signal, a decent quality numeric pad should work. You may want to spend more time checking why it doesn't before investing in the wired switch.