In general, if you see the test banners in Test Mode, then your integration is likely correct. The most common cause of not getting banner ads is lack of available banner demand. This would be expressed as “No fill for placement” errors.
Many of our banner advertisers will only bid into games that have implemented the app-ads.txt file correctly. This file is a standard that has been developed over the last few years to combat fraud and create transparency in the advertising market. More info can be found in our documentation: https://unityads.unity3d.com/help/resources/app-ads-txt-support
However, even with app-ads.txt implemented, the availability of banners will vary greatly by region. We continue to ramp up banner demand, but a lot of that improvement has been focused in specific regions, so not all countries are seeing banners available.
If banners represent a major issue for you, I would recommend investigating other options. Unity Ads is supported in all of the top mediation platforms, and many do have support for our banner ads: https://unityads.unity3d.com/help/resources/mediation