When you first open the back shell, please take EXTREME CARE not to simply pull off the back shell directly, otherwise, you risk irreparable damage to either the 900 MHz Antenna cable, the socket on the main RF PCB board it connects to, or both of them, thus rendering the TANDEM feature inoperable.
The rear Shell needs to be VERY carefully separated and great care is taken to carefully unclip the 900 MHz Antenna.