I took it apart so will try it later. Does it matter which half of the 8 pin I use?
also if you plug in a psu and turn it on out of the box and not connected to computer would it turn on?
Cables and ports are keyed to only fit one way; do not force anything.
There is a latch on the psu cable that should match the latch on the motherboard 4 pin connector.
The 8 pin will fit on one side or the other.
What would be your objective in plugging in a psu when it is not connected to anything?
It will be in standby mode, but that tells you nothing.
Do not bother with a cheap psu tester.
They can only confirm a dead psu, but will say nothing about proper operation.
What is the make/model of your 550w psu?
If it is a cheap unit, you may want to return it.
A cheap psu can be dangerous.