All of the above did not work for me. Cloning the repo was working correctly but I was still getting an unrecognized import error.
As it stands for Go v1.13, I found in the doc that we should use the GOPRIVATE env variable like so:
$ GOPRIVATE=github.com/ORGANISATION_OR_USER_NAME go get -u github.com/ORGANISATION_OR_USER_NAME/REPO_NAME