iPhone doesn't start after battery replacement

even if your old battery is bent, doesn’t necessarily mean it’s broken, you can still use it for testing to cross check the new battery.

reconnect the old battery (provided it’s not damaged from the removal), reconnect the screen ( you don’t need to put the bracket plates back on). if you can, put a piece of corrugated cardboard (or use anti-static foam) between the screen and base to prevent shorting and you can still turn the phone back on. you don’t need to completely re-assemble the phone to test it. this way you can at least see the what the phone does with the old battery (provided it still works).

if it works with the old battery, but not the new one, other than jumping to “it’s a bad battery”, i’m not sure what to tell you. personally, i’ve never had a battery issue cause problems like this. one thing to try, if it’s not already at the latest IOS, maybe upgrade it before trying the new battery again.