iPhone 7 shuts iff

I have a iPhone 7 that I bought new from Walmart ended up cracking my screen but it was still usable then a some point i took it to a local phone repair shop and I got the screen fixed but ever since then my phone will shut off at random only turns on if plugged in and will it only stay on for like 5mins top at a straight use battery shows charging or full power when I turn it on for the first time then after a few minutes it shuts off when back on it shows dead battery at 1 % I’ve restored phone using iTunes doesn’t change I’ve put it in DNF mode same thing no change