Car stalling after alternator change

So I recently had some issues with the alternator not sending charge.
I have bought a brand new OEX replacement. All went well with the change over.
When I go to start it, the car starts fine although she won't stay on. Turns itself off once the idles drop.
I can keep it going although I need to keep the revs up.

A few extra notes.
Battery has been disconnected for approx. 3 weeks now.
The battery is now fully charged.
Voltage increases to 14> Volts when car is running (With revs up). So nothing wrong with the alternator.
Car is Nissan Patrol GU 4.8L 2002.

I have been told that the ECU may have been reset since leaving it disconnected for some time. Driving it around will re-configure itself out.

Anyone know or have any insight about this problem?
Can I just drive it around and it will sort itself out?
Can I manually reset the ECU?
Is there another way to fix this without driving it around?

When you say the battery is now charged, how did you charge it and test it?
Even though the new alternator may be sending 14VDC+ , the battery may still be stuffed.

@GQtdauto is having similar problem, only rob's stops and needs to be restarted

With a new Alt and a fully charged battery (car won't start if Battery is stuffed so if it cranks and starts then should be OK as well) the idle volts should be 13.8 or better... If it isn't then you still have electrical gremlins