iPhone 6 Plus apple logo keeps flashing on and off.

My iPhone 6 plus wont turn on. When I plug it into the computer, iTunes doesn't recognize it. When I hold the on/off button and the home button, iPhone turns on and the logo appears. But if I let go of either buttons, it turns off immediately. However, since the screen isn't sealed into the iPhone yet (because I did a battery replacement to see if the problem was the battery), I can see that although the logo went away and the screen is black, the LCD is still lit. I can't get it to turn on Recovery Mode to restore it or update it. What is possibly the problem?

Thank you in advance!