Nullmodem *is* serial. You need a serial nullmodem cable to connect two computers (similar to a crossover ethernet cable).
To connect a computer to a device (e.g. a modem), you need a normal (non-nullmodem) serial cable.
You cannot connect two computers with a normal serial cable.
As you've got an Amiga with PCMCIA, best would be an ethernet network card (3com or NE2000).