I got rather excited there for a moment at the prospect of Unity banners! My understanding is that there is no support in Solar2D for Unity banners. From the docs:
“Unity Ads video interstitial; rewarded video”
I truly wish there was support for Unity banners (unless I’m mistaken and there is support for them?) as AppLovin banners are broken. See my post here: AppLovin Banners
Maybe they will be added at some point in the future?