1998 Isuzu Trooper Keyless Remote not unlocking all doors

1998 Isuzue Trooper, power lock, windows and sunroof. Keyless remote will only unlock the driver side door and not all four doors. Same thing with locking. However, the manual lock/unlock switch works on the driver side (unlocks/locks all doors) but not on the passenger side (will lock/unlock passenger side only). Maybe the passenger side is normal behavior.

Battery was replaced a few months ago, but as this car was a gift from family, we are unsure if the keyless problem occurred before or after the battery switch.

Have tried reprogramming the keyless remote with a HowTo I found elsewhere but either I am doing it wrong or the directions are incorrect because there is not change in functionality.

Since the door switch works on the driver side (unlocks/locks all doors) I do not think this is a mechanical issue but then again, I'm not a Trooper expert.