Hi Vitor,
Regarding
“Syncfusion's Toolbox doesn't appear. is there any way this can be fixed?”
Currently there is very limited support for setting properties for the custom controls in Xamarin Designer for iOS.
Supported properties are numeric type, enumeration type, string, bool, SizeF, UIColor, or UIImage.
Hence it is not possible to render our custom controls in Xamarin Designer for iOS (needs object type properties like ItemsSource etc.) even though it will appear in toolbox.
Link: https://developer.xamarin.com/guides/ios/user_interface/designer/ios_designable_controls_overview/ Please let us know if you have any concerns.
Regards,
Saravana Pandian M.