Why won’t my PS4 Pro turn on?

I bought a used PS4 Pro from someone. When I got it home and plugged it up, it wouldn’t come on. Sony, of course, would not do a thing for me since I bought it used from someone, so I did my own research to try to troubleshoot the problem. I finally determined that it might be a faulty power supply unit and took it in for a new one. The technician agreed with my theory and ordered a new PSU, but he called me a few days later and said that it still wouldn’t come on with the new PSU and that it was likely an issue with the motherboard. He proceeded to tell me that he does not do that kind of repair, so it’s back in my hands to fix it. At this point, I’m at a loss. If I knew the motherboard was the issue, I’d go ahead and buy one since it’s cheaper than a new console, but I don’t want to shell out $150 to “fix” this thing when I’m not even sure that’s the issue. Can anyone provide some tips on getting it to come back on when the PSU isn’t the problem?

Also, I have already taken it apart and looked for cockroaches and dust. Everything looked fine, as far as I could tell.