Rather than checking between N1 to ground and N2 to ground, check from N1 to N2 looking for 240 Volts.
Try this: Turn off the utility main circuit breaker. Put the generator in auto and let it transfer. Measure from N1 to N2. You should see zero volts. If you see voltage your N1 and N2 wires are connected to T1 and T2 or E1 and E2. Correct the wiring.