Hello looking for some help

My car will not start, I turn the key get a click then two error messages on the central display.

1. "Immobiliser active"

quickly followed by

2. Engine Malfunction

I am working on the first error. so far I have tried

1. Jump starting

2. Replacing the battery

3. Using both keys in case the main key fob was defective.

4. Removed the +flying lead from the battery and earthed flying leads to chassis to remove any static or logic stuck in the ECU

5. Tried locking the doors and unlocking them.

6. Left the battery removed overnight.

7. Checked immobiliser fuse and it is intact

All of these did not work , run out of ideas, Should I track down the ignition immobiliser and see if it is defective.

Advice welcome