Android+ & Application Catalog
I have had this issue myself. It was related the Device Management Address Port not being open and accessible.
I had to use a device side VPN to bypass this issue.
Hope this helps.
The app catalog is hosted by the DSE which is an extension of by the Deployment service and will have the address of Device Management and port. The Android device/MC Agent needs to be able to access this URL for the AppCatalog to populate. Typically the pattern of the address would be something like:
https://<FQDN of MC>/mc/mdm/appcatalog/json/<device id>