Could it be so simple that the USB-flex cable isn’t plugged in properly to the motherboard?
So when you put some good amount of pressure at the bottom of the phone it gets a good enough connection to make it charge.
From your question i’m not sure if you want to repair the device yourself or are just asking to see if it’s repairable.
If what i’m saying is true you would only have to remove the back cover/glass from the phone, which is glued.
Then remove a few screws , remove plastic bracket and then plug in the USB-port connector to the motherboard.