If I plug charger in the phone turns on but isn't charging at all.

Hi. I've bought a dead 7 Plus with no signs of life, I've put a new battery in that had juice in it. This allows the phone to boot onto the home page with no issues, I started testing everything. I plugged the charger in to test the port wasn't dicky and it didn't come up saying it was charging, i figured ah huh! Here's the fault it needs a new port. I've since replaced the port with a known working port, put my battery in that has juice in it and the phone powers up as normal. I then try and plug it in the charge and still now charging symbol coming up or any sign of it recognizing the cord has been plugged in… the really weird part I've figured out is if I turn the phone off and then re plug the charger back in the phone will boot up because it's been plugged in!? But once it turns on it doesn't show that it's charging or have any signs once again that it's charging. Have tested multiple cords to no avail. Every single time I turn the phone off and plug it in to charge it boots up on its own from simply being plugged in. Any tips or knowledge on why this is happening please help! :)

Edit: I've pulled the board and charging port out of the housing. Ive just had the motherboard sitting on my desk with the charging port, battery and a screen only connected. Plugged it in to charge, its turned on and done the exact same thing so that crosses out my charging port not being aligned properly :(