Assuming that it worked before you changed the battery try this.
Remove the battery from the plip (fob) again.
Press each button for at least 5 seconds.
Refit the battery.
Unlock the card using the key, then operate the lock button at least four times.
The car remembers the plip not the plip the car and so changing the battery should not case what has hppened to you. However if the above does not work you may need the plip to be resynch'd to the car or even a new plip. In which case you need to go yto a LR dealer or and Independent LR dealer who can both resynch with a diagnostic omputer or supply and resynch a new plip. I know that plips aren't cheap but I suggest that you have two so that you can use the car under these circunmstances because the immobiliser's transpodner is (as you have found out) in the plip.
