My iPhone 4s has had terrible Wifi connectivity ever since I got it used a couple years ago, and now a worse problem was starting: the battery was being used up very fast. Looked like time to get a new phone, but they are all so expensive. I figured it was worth trying to fix it myself, spending $60 to get maybe another couple years out of the phone.
Repair was a lot easier than I expected. I followed along on a MiamiBeach youtube video and did the Wifi antenna repair and battery replacement at once. It was very easy. Only, I elbowed a tiny screw off the table and never found it. Luckily it was a second screw holding down a small part and I don't think it was truly needed.
The Wifi is a lot better now - almost totally reliable where before it almost never worked at all. The battery is a bit better but not totally. I think it must be leaking power somewhere, and I'm not sure how to fix that. Indeed, the repair might be for naught.
Don't lose the screws - put them in a teacup or something.
Fast battery loss might be due to a leak, not an old battery!