The problem here is that you're using a built in TileSource and simply providing 'already cached' images for that TileSource.Hallo Martin,
I store my tiles in folders(osmdroid/tiles/Mapnik) up to zoom Level 18, not in a zip Archive.
I don´t choose a TileSource
settings : Mapview1.SetDataConnectionEnabled(False)
So what is to do for zoom > 18
And the built in Mapnik TileSource is hardcoded to have a maximum zoom level of 18.
You cannot change that hardcoded value.
Maybe you could create an XYTileSource with a higher maximum zoom level...
The example code in the linked to thread could be modified:
MyXYTileSource.Initialize("MapnikOffline", 19, 22, 256, ".jpg", "http://localhost/")
But before requesting tiles over HTTP the new XYTileSource would check for the existence of cached tiles at osmdroid/tiles/MapnikOffline.
So put your offline tiles in that folder instead of the default osmdroid/tiles/Mapnik folder.