Hello Matthew,
looks like you bought a used phone. A used iPhone sometimes can be a problem. Unfortunately, thanks to the era of Youtube and other similar services, there are millions of "electronics" in the world with a knowledge level "below sea level". You can fall over laughing how some "mechanics" can be creative to earn some money. I don't mean putting the chocolate wrappers inside.
Your phone is falling apart after being repaired first. Because someone used the wrong alloy for soldering, bad temperature, pulled out a few elements because it was difficult to de-solder them (and He did not complete the lack, because it would be difficult to install again. Anyway, if it works, it probably was unnecessary anyway - typical thinking of idiots), he did not know, did not see etc. My advice, find a really good service, it may turn out that for little money someone smart will repair your phone and it will work many years. If you would like to do it yourself, sometimes it can be a perfect fun. However, you must remember that additional hardware is necessary, if you do not use it, you will make the same mistakes as the person who previously repaired your phone.
I provide a blog on this topic. Use a translator because I am writing it for my local community in EU. There you will learn more about the process of repairing your model, but also about other similar cases like yours, related to dishonest and uneducated repairmans.
regards and fingers crossing. If you need any help, please write. I will try to help you remotely.