If you experience "no backlight after screen or display replacement" or "very dim screen after screen or display replacement" it usually means you made a mistake or the screen flat cable had an exposed contact point that resulted in damage to the back-light circuit that requires micro-soldering repair using the proper diagnostic and soldering tools, and of course proper experience.
To confirm that your screen is not itself damaged, try another known good screen on your dim phone (or the dim screen on another known good phone, but those exposed contact points may cause the same board damage to the other phone).
MOST IMPORTANT: make sure you power down the phone then disconnect the battery before performing any kind of screen swap.
If you confirm that the screen is working fine except on your phone, your local independent repair shop (not an Apple Store, they don't offer this service) may be able to repair it for you if they are experienced in micro soldering. Or they have it outsourced to a specialized micro soldering technician. You can find a list of those on a link on my profile. Most if not all of them accept mail-in repair.