“…. it will only display externally if the lcd cable is disconnected.” This may be due to Windows recognizing that there is no primary display i.e. cable disconnected, but that there is an external monitor which has been detected. Did you have to manually switch to the external monitor using the function keys Fn + F9?
The following is just a theory I have about the problem, but maybe worth looking at all the same, if only to eliminate the possibility.
With the laptop on and the laptop’s LCD screen connected to the motherboard and no external monitor connected, try shining a torch (flashlight) at an angle close to the laptop screen and check for images e.g. Windows desktop.
They will be very faint if there are there so trying in a darkened room may help to see them.
If you can see them then it is a backlight problem.
What might have happened, is that originally the backlight power supply fuse on the motherboard may have blown due to say a fault in the LCD panel or the LVDS cable (damaged where it passes through the hinge??).
If say the fault was in the LVDS cable, then changing the screen or cable wouldn't help as the fuse has already blown.
If the new motherboard was tried with the original LVDS cable first connected and not the replacement cable then the fuse may have blown on it as well so the problem is still there after changing parts after this as now both motherboards have been affected.
This might also apply if the problem was in the original LCD panel and it was connected to the new motherboard first before the new or old LVDS cable. I don't know the sequence of the trying of the replacements i.e. which came first when trying the new motherboard - original LCD panel or original LVDS cable. If it was both the new LCD panel and the new LVDS cable then it should have worked as all parts were new.
Check both motherboards for the location of the backlight fuse if it has one and check if it is OK with a DMM. It may be near the LVDS cable connector but maybe not. You would need the schematics to know for sure how the backlights are powered.
What is the motherboard's board number - printed on motherboard?
Did @brettpocalypse mean an Asus GL703VM-NH74 and not an MSI laptop?