Replaced logic board, 4th LED not turning on, please help!

Well my late 2014 imac with 4gb GPU and i7 processor just didn’t start one day. Checked and replaced Power supply but only got 1 LED to light up on logic board still, so ordered a used on en aBay. The one I ordered only has a 2gb GPU but I figured this wouldn’t matter, since it’s also an EMC 2806.

Before installing new used logic board I replaced the battery on it.

Re used existing SSD drive that sits on the logic board (512gb flash).

Reinstalled everything accordingly, very carefully, with anti static protection.

Now LEDs 1-3 light up but LED 4 does not. Checked connections to the LED, all good. Checked with external monitor, no dice. USB does not work either, it powers a hard drive but does not connect to keyboard (CAPS lock key has no effect, doesn’t light up).

Tried all the SMC resets and PRAM resets but nothing works , keyboard doesn’t work so all useless.

LED screen never turns on.

Do I have another bad logic board or is there something wrong with my setup here? I thought I could switch to a 2gb video logic board with no issues. Going to try another used logic board, but anything else I should look for?