Why does my iPhone 6s not turn on after battery replacement?

So I bought an iphone 6s battery replacement kit, it worked great as soon as I put the battery in and turned it on it worked. On the box it said to use my iphone until the battery goes to zero, then after that leave it charged until 100% and charge for 2 more hours once it reached to 100%. I did have to unplug it at some point during its charge because I had to go to school. When I tried recharging it I saw that the battery was losing charge instead of gaining and then once it reached 1% it shut off, Ive had it plugged in for so many hours in fact more than a day. I've tried force starting it by holding down the power and home button at the same time, holding the power button for 3 seconds and then hold the home button, reopening the iPhone, and check if all the cables are plugged in including the display cables. All of that just to get a black screen, not even the battery is low screen just a blank screen. Should I return the battery kit or try another method I don't know what to do at this point. I know the display is working because it did turn on while I was charging it at school.