LPT: Turn off cellular data for any games that are "free". The game uses your data to download ads, meaning you are actually paying for them.

This can apply to almost any app that you don't need internet access to use, but games are the most common. I know that you can do this in iOS and I assume it's possible on Android.

There is an argument to be had that the developer provides the game for free and subsidizes this by using ads. However, this comes at your personal expense if you have a monthly data limit - you're wasting your data on something so another person can profit. If you play the game on WiFi, then the developer will receive revenue from the ads downloaded during that session.

If a game doesn't let you play without an internet connection, then that's a different conversation about whether or not you should be playing that game at all.