When I download multiple files at once the downloads randomly stop

Can’t give you enough hearts. Couldn’t figure out for days what was wrong with my setup (Rock Pi 4 Model C with Penta SATA Hat, Apache2, Php 7.3).

Synchronisation (Download from Server) always stopped after some minutes. There were no issues while uploading (200GB) whatsoever.

After connecting via a LAN adapter it (almost) works like a charm.
Many thanks!