Having never used this software before, my guess would be your VPS provider is limiting the upload speed of the VPS. The data would be uploaded slowly to you, which means your download speed is limited as the client.
Meanwhile the upload speed is reported as being high enough, since the VPS can download your data at 50mbit/s, as this cap is higher.
You can go to the desktop mode and install the GOG launcher. Then after installing your games, open Steam and add the games as non-Steam games. They’ll show up in game mode as well.
The Heroic Launcher can add your games from GOG, EGS, and some others iirc to Steam automatically. I believe there are other, supposedly better options too, but I can’t remember the name(s) right now. Also don’t remember what (if anything) was wrong with Heroic.