Just to formalise my bit, if SRA are hosting on a GCP bucket, Google will not permit ftp access which is at least part the reason behind your question. 

They do have their own ftp equivalent called `gsutil` which will permit direct download if you know the bucket name. `gsutil` is part of the ClouD SDK package and can be downloaded via conda/anaconda `conda install gcloud-sdk`. This will give you command line access if you know the bucket names. There is a past post on this BTW.