Alarm system not working! 2000 Golf

Anyone have any experience with this? The alarm was armed and would not disarm when the car door was unlocked and the engine started. The alarm would go off and ultimately I had to disconnect the battery to get it to stop. The FOB seems to have no effect now. Put new batteries in it, but not sure if it’s working. The car starts fine and can be manually locked, but the alarm does not arm. Would this be a FOB, door latch, or alarm system problem, and any ideas how to trouble shoot and eliminate some areas. The FOB is no longer available from the VW dealer.