This app was mentioned in 3 comments, with an average of 1.00 upvote
Data eye - I've been using this because Google services was using lots of data. It used up a 1gb before I noticed.
Lot of Cloud apps don't really sync in the background (until you open the app and browse to the file you want to download), and you can also restrict background data and even use 3rd party apps to block some apps from using mobile data, like DataEye:
https://play.google.com/store/apps/details?id=com.lotusflare.datasensor
On the other hand, if you have OpenSSH server running on your old PC, you can use FolderSync which supports syncing over SFTP.
I'm new to android. Whilst searching for apps for this same problem, I found this app called DataEye . Have a look :
https://play.google.com/store/apps/details?id=com.lotusflare.datasensor