Door lock remote control synchronisation

[privateselling]

I own a 1999 Boxster. For an unknown reason it is impossible to (un)lock the doors or the trunk with the remote control although it worked perfectly until some days ago.
The battery is OK and when I push one of the buttons then the red signal light in the key holder starts flashing but nothing happens.
My spare key works very well so I am pretty sure that the central door locking computer is OK .
I think the first key is not longer recognized by the central door locking computer.
Of course I replaced the remote control battery by a new one (3,2V) unfortunately without success.

1. Is there any possibility to 'synchronize' the key with the central door locking computer without using the special Porsche tooling (PST2) and how can it be done ?

Would appreciate your experiences / advice.

Alfred