How to determine the reason my tablet isn't charging?

I have Mi Pad 1 Mocha, since a week ago it can’t be charged. The battery is fine since I’ve just replaced it 3 months ago with original battery. I’m guessing it’s the charging port but I’m not sure. I’ve tried changing the charging cable and the cable works fine on other device but not on the tablet.

I have to hold the cable down very hard in order to charge it, but after 3 minutes or so it won’t charge again and I have to hold the cable to different position. How to determin what part I should replace? Or does the charging port only need tightening?