Problems after installing new logic board

I don't know if I shouldn't post on a new thread, since I am not the OP for this one. Please let me know.

The new logic board arrived and I have installed it in the mac. Everything ok until this point.

Where I started to have problems ....

As the built-in display is cracked I can't do anything, but I knew that from the start. So, I connected the macbook air to the external display and the problems started to happend here.

I set up the macbook the following way:

- I connected the external power adapter, connected an external keyboard and mouse and connected the external monitor.

1 - I can close the lid plug/un-plug the monitor, do a million of different combinations, but it didn't wotk work. The clamshell mode is never activated.

2 - I tried several shortcuts to extend the monitor and nothing.

3 - I shut down the mac, closed the lid, removed the power and put it back.

The mac started automatically and I have image on the external monitor, but it's extended instead of mirrored, so when I press "D", it won't show the menu.

Opening the lid reveals the LCD black. It's, basically, off. Removing the cable of the external monitor does nothing, the built-in display continues off until I reboot again.

I don't mind this, since it will be a headless computer.

Question: how can I force the external display to be the main one, so I can configure the computer?

The previous owner formatted the ssd , so I get "?"and cannot go any further regarding booting the system.

Kins regards,

Pedro