Asus G751JY won't recognize AC adapter or charge; how to diagnose?

My laptop, an ASUS ROG G751JY gaming laptop, worked perfectly for about 3 years, then all of a sudden last week it would stop recognizing the charger when it was playing intense games (i.e. when hot). It would start charging again after a reboot or sometimes if I unplugged and replugged multiple times. I took it apart to clean the fans, which lowered the temps a little bit, but didn’t really stop the problem. Then a day or two after that, it just stopped charging altogether. It still turns on but since I can’t make it charge, the battery will die soon.

I’m not sure how to diagnose this. Bad charger? Bad charging port? Bad motherboard? I saw on ASUS’s ROG forums that many people have trouble with their charge port but the cheapest one I saw online that didn’t look like a scam was $110. I don’t want to just have to guess and end up buying the wrong thing, as parts seem to be pricy so I want to make sure to get the right thing. I don’t have any multimeters or anything either. Maybe someone more experienced can guess as to what the problem is?