Why won't my 3DS boot or screens turn on?

I bought an og 3DS for parts from eBay. The problem with the console is that the 3DS won’t boot and the screens are blank (as in not even the backlight turns on, they are just completely dark). The description from the eBay listing didn’t say anything, just a sentence from the owner saying it wouldn’t turn on. When I first got the system, I noticed that it wouldn’t power on and checked the battery’s voltage which was completely dead so I replaced it with a working one. The console powered on (as shown by the blue LED light) and can charge but nothing pops up on the screen, not even the backlight. The system doesn’t power off after a few seconds and stays powered on indefinitely so it’s not a booting issue related to a boot.firm as far as I’m concerned. Another thing is that the L button came broken so I got rid of the L button ribbon.

What I’ve tried so far

1. Using arm9loaderhax.bin file in an SD card and a boot.firm. Someone I know recommended I do this since booting issues are common in cfw 3DSs (I dont sell cfw systems but I often get stuck with them since some sellers end up messing with their system or lose an SD card so they can’t figure out how to boot it up and they end up selling it for parts). No change.
2. Wifi-card trick. I opened up the console and reseated the wifi-card. No change.
3. Cleaned it up superficially. The system was very grimey on the edges of the housing and on some parts of the motherboard such as the game cartridge slot. The motherboard wasn’t in bad condition as far as I could tell since there was little to no corrosion on it (Im a novice at this so I could have miss some tiny liquid damage).The grime was dark red. I used some cotton swabs and 70% isopropyl to clean it up. No change.
4. Checked for any damaged ribbons. The ribbons looked fine and the inside looked untouched. I reseated some ribbons and checked the contacts and none looked damaged (aside from the already broken L button)

What could be the issue? Could it simply be a screen problem or am I missing something that I’m not understanding? To summarize, I have an og 3DS that won’t boot, stays powered on indefinitely and screens don’t turn on at all.