Question Weird issues with HDMI/DVI

Apr 3, 2019
This is pretty unusual. I have a prebuilt computer (tc-885), and it came with a graphics card preinstalled, a gt 830 (?) and a i5 8400.
After upgrading ram to 2x8gb, and changing the power supply, the integrated graphics and the gpu both seemed to work fine on my hdmi cable showing the full 75hz display.
Now, after getting a gt 750 from a friend (i know it's working, it worked in his computer) none of the display ports seemed to work- i checked the integrated graphics just to ensure it wasn't defaulted,
but, it still seemed to not have any connection. I quickly gave up on the gpu, thinking it was just broken, however, as i got a new gpu, an rx 570, there was the same problem.
the hdmi cable didn't work. So, instead, i tried a new monitor. instead i used a dvi to hdmi, which worked fine, albeit at 60fps. I switched back to my old monitor and it works! but only on 60fps.
now, attempting to plug in both cables, at the same time, to try to be able to switch from one to the other to test if it works, one display (hdmi to hdmi) didn't work. Display says 'no signal'.