Not charging and takes a while to boot

My iPhone 6 bought on eBay came without a screen which was an easy replacement but then the battery was bad so I also replaced that and after that it was working. I decided a couple of weeks later i should apply the adhesive under the battery because it was working great and i could sell it. When i went to turn the phone back on it took a long time to boot up and when it booted up the battery was at 1%. So i went to plug it in and it wouldn’t charge. So i went to go plug it into my computer and the computer didn’t recognize it in recovery mode. I have had this same exact problem on a 7 plus about a week ago and i have taken it to the apple store to try and fix it and they told me that it was on the logic board. So I decided to solder a new connector on the 7 plus and it still didn’t turn on even when connected to my computer.