Sync conflict, downloading instead of uploading

Nextcloud version (eg, 12.0.2): 15.0.4.0
Operating system and version (eg, Ubuntu 17.04): Synology NAS i think
Nextcloud Client: 2.5.2 on Linux Mint 19.2

I try to backup files to a nextcloud server of a good friend of mine. the connection is bad, and it is quite a lot of data, so it gives errors from time to time that a connection got terminated.
That wold not be a big problem, for itself, as it would try to sync it again the next time a sync is started. But it takes so long that a sync was never fully done since it syncs literally for days until the client hangs up. Afterwards, the client trys to upload some files as it should, but then switches sometimes to download copies of previously uploaded data, which takes up much time and is useless in general, because the data folder is completely switched out by my backup mechanism (rsync with hardlinks) on a daily bases. It got so bad that most of the time i am downloading files instead of uploading, while those files are deleted after a day and it starts anew.

So what i would need is a way to force the client to upload (client -> server) instead of downloading (bi-directional). I use FolderSync on Android to sync some files, and i can tell it to either go client>server, server>client or bidirectional, is this possible with the client, too? this should be an option in the client, but i didn’t find one, maybe there is something i can change in the config-file of nextcloud client?

(If I should post this somewhere else, please say so)

greetings

STonE