It is possible to use private CocoaPods, Maven repositories or submodules, but you need to ensure Buildozer has access to them.
If you are using SSH keys to access your main repository, then the cocoapod or submodule needs to be accessible using the same SSH key. Also, if you use SSH keys, your submodule or repository urls should use this format:
Buildozer wouldn't know to use the SSH key in the second case. In the first case it knows you are trying to use SSH, and will use an SSH key agent to ensure that the checkout of the private module is successful.
Similarly, if you are using credentials instead of SSH keys (not recommended), then you need to use the second url format.