New Battery not charging

Bought a new battery for my iPhone 6s. Arrived on time in the nice package. The replacement went without any issues. After I finished turn the phone on and everything was working perfectly. The percentage on the battery was 51%. So I decided to charge fully before I use it. After charging for about a minute or so the phone shut down and restarted and the battery went down to 21%. After that I just let it charge. About 6 min in to charge it shut down again and restarted and charge went to 29%. And after that I still didnt take off the charge and let it continue to charge. 30 minutes later it was still 29% and didint go up.

So if anyone can give me any direction what do next.

Thank you in advance.