Faulty phone

Bought a Samsung J7 prime on the 8th of December 2017,cash R4 300. Now this phone has been in for repairs twice already. The first book in was because it would freeze as soon as the battery would be below 15% till it switches off. Then it came after 3 days it was worse. I don't know how many times I have to restart the phone.Also ,the return button sometimes didn't work.It got booked in for 2 weeks then I got it back. I'm experiencing the same problem. IT'S STILL FREEZING. I'm so upset because ive never enjoyed this phone.