iPhone overheats while on mobile data

I have a somewhat peculiar problem: a few months ago, my iPhone 6 started overheating for no reason, and my battery was draining faster than I could charge it. At the time, I thought it was just the battery itself, as the iPhone was 3 years old.

Recently, I purchased the iPhone X, and after setting it up I notice the same problem, battery draining completely after 4-6 hours and overheating. After a little check, I found out that it was only happening when I was using mobile data, while on WiFi everything seems to be fine. Anyone has an idea what could be causing it?