first of all, great looking car.. welcome
there are two cables connecting the shift lever to the transmission.
one is for the left-right, the other is for front and back motion of the shifter.
if you go to here
Porsche Original Parts Catalogue - Parts and Diagnostic Information - Porsche Service - Dr. Ing. h.c. F. Porsche AG
you can see the catalog for boxsters. select your model..
my best guesstimate is
it's the case of broken/disconnected "fastening part for cable adjuster" by the shifter or the "linkage adjustable" pieces by the transmission.
easiest is to look between the muffler and the secondary catalytic on the passenger side. there is an aluminum protection plate and the cable ends are behind it.
make sure that the parking brake is on, car is on level ground and place brick or 2x4 such that the car does not move.
get a person to move the shifter left-right-left and see if the upper linkage at the transmission moves.
if it moves then follow the linkages and see if it is moving a shaft in/out of the transmission.
if there is no motion, most likely is the cable adjuster by the shifter or a square block in the shifter assembly.
anything else... is... beyond the understanding of this Boxster owner.
hope this helps.. and good luck.