Couple of points - was there a second battery in the bell/strobe unit that needded changed also ?
Tamper is simply power out from the alarm board to a detector such as a PIR and then back to alarm board if this simple series circuit is broken it will come up as Tamper - open the alarm panel this will itself cause a tamper fault as the cover most likely have a tamper switch fitted.
Locate the circuit involved ( 6 ) find the negative 6 or 12v terminal and locate the return tamper wire from the PIR - you should be getting 6 or 12v between the two, if not then the circuit is broken and you need to find out where.
Could also simply need a factory reset which isn't as simple as powering it down and disconnecting the battery and re energisiing it. There is a procedure for it and it has to be followed.