Ideally you would have the system serviced annually and as part of that the batteries in all the devices would be replaced as required, I assume there was a reason for cancelling that?
ADT Visonic systems use components that run on a different frequency so support for replacement parts would be limited to ADT without buying second hand.
While it's possible any engineer maybe able to get you sorted if it's simple or just batteries, but your going to get limited support and may not be able to get the app back online.
One thing that may point you in the right direction if you keep pressing the "i" button it should point you towards the faults stopping it from arming.