I tried what Rob suggested, but I can only get admob and house ads to show up.
If you look at the admob documentation, they talk about adding “mediation adaptors” to your app and configuring your AndroidManifest.xml file. See this page for example:
https://developers.google.com/mobile-ads-sdk/docs/admob/android/mediation
Does the admob plugin include the mediation adaptors? Does a build produce the correct xml file?