As of right now, there are a few ways to put ads in your mobile game, but it is not very easy to do it for free.
In the past, there was a free admob plugin on the marketplace, but the creator removed it and made it $50. If you got the free version, you can still use it, but not in the newest version of unreal engine. thet $50 plugin is here: https://unrealengine.com/marketplace/en-US/product/easyads-pro
The same creator has made 2 other ad plugins. One for unity ads, one for Vungle ads. His profile with all of his plugins is here: https://unrealengine.com/marketplace/en-US/profile/xufei+wu?count=20&sortBy=effectiveDate&sortDir=DESC&start=0
I use his vungle ads plugin, and it works great.
There are also a few more plugins you can try. There is a chartboost plugin, but you can only use it on up to Unreal Engine 4.16 (this is a old version of unreal engine.)
There is also a $100 plugin for ironsource. I have not used it, and as of right now there are no reviews, but you can try it here: https://unrealengine.com/marketplace/en-US/product/ironsource-mobile-ad-network
These are all the Unreal Engine plugins for mobile ads as I can tell.