iPhone store 3-weeks long battery replacement

I took my iPhone 7 to iPhone store Eastgate for a battery replacement as my battery was depleting quickly. They said because my phone was not purchased in SA, they would hold it for longer. I had no choice but to book it in.

In about 3 weeks I got my iPhone back but the battery performance was still the same. I went back and reported this and they said I would have to leave for the same length of time again, and the consultant switched off some applications that he suspected were consuming the battery quickly, and asked me to check. I use my phone to do my work and without it, my work data gets distorted hence my reluctance to just hand it over for 3 weeks for them to check, possibly returning it after 3 weeks with the same issues. There’s no other places I know where my phone can be battery replaced and still be fit for iPhone or Apple warranty

I have written to iPhone or apple SA a couple of times and they have not responded. Please help