Just as it written above.
Unreal Engine 4 is a great engine, no doubt but without working ADS system there is no any reason for a blueprint coders develop anything for mobile platforms. The problem is that all plugins at marketplace is broken or out of support. And your built in system is not working as well. UE4 AD system does not support reward and native ads. Thats a shame.
Again: we are in 2020 for now. And there are no working built in ADS System just as no Reward Video, Native Ads.
You want to overhelm Unity in mobile gamedev? I guess that s wonderfull starting point.