On my T16 powered +4 you needed to send the ECU & the immobilizer plus new fobs to a Rover technician to set them up or you can disable the immobilizer connections to the ECU.

I had to replace the ECU on my T16 and the replacement was set up to ignore the immobilizer.

JohnV6
2022 CX Plus Four
2025 MG ZS EV aka Trigger