IPhone 6s uses lots of data and battery quickly runs low: how to fix?

I have an iPhone 6s that is perhaps 2 years old. For the past 6 weeks, it has been using a lot of data if I use the Internet using cellular data (rather than Wi-Fi); for example, browsing a few sites last night used 221 MBs of data in about 4 minutes. The battery also very quickly runs low.

Is there a way to fix this?

This happened in the past a few times when I had used a lot of data (not realizing that I needed to shut off cellular data and use Wi-Fi only), but the issues always stopped within a few weeks, perhaps once that billing cycle was done.

This time, however, I used a lot of data in March and the issues are still going on, more than a month later.

The phone has nothing to do with the amount of data you use on cellular. It's more than likely the sites you're visiting are filled with ads and videos which uses a lot of data. To fix that I'd recommend a browser that can block/reduce these things such as Opera or Dolphin. They both have ad blockers and Opera has a data saving option.

As for the battery dying quickly, the 6S is somewhat old now so your battery probably isn't as good anymore. A battery replacement can fix that, I got mine Replaced for £15 at an Apple Store and the battery life has improved greatly.

Data use is up to you. As far as battery maybe maybe it needs a new one since 6S is from several years ago (the apple store can put a new one in pretty cheap, and it will be like new again)