Hi Pinete,
Last night I did have some success with this after more testing but it can still seem quite unpredictable. I've found that it can take even several hours for the "test mode" option to take affect on Chartboost, during which time it didn't seem to serve any ads. I won't really know how good the fillrate is or if it works with production ads until deployed.
Mediation is still very inviting since Google supports around 30 ad networks for banners, interstitials and rewarded videos with very minimal work required. Through Google Admob mediation it can automatically monitor the eCPM of all added networks and serve the most profitable ones first.
I will have to research what version of Google Mobile Ads / Google Play Services SDK is being used with AGK. The last mention of Admob SDK was way back in version 2.0.22 from 2016 which used 7.15.0, but hopefully it has been updated as there have been 16 major releases since then including ones compatible with iPhone X etc. I feel in the dark about the third party SDKs with AppGameKit as it is rarely noted when they are updated and a lot of the code is compiled and not readily apparent.
For this project I am now checking for both Admob and Chartboost rewarded videos. If none are available I also check for interstitials as a fallback and will still reward a small amount when those are clicked even if we can't track that. Again mediation would really be the way to go as it'd be a single banner/interstitial/rewarded ad call through AppGameKit but the best network would then be served up automatically. I will still experiment to see if I can get mediation to function as I had it working with banner ads a few years back and wrote a tutorial about it here.