I’m having a similar issue, but at about half the speed – 400/20 connection, downloads at 50MB/s+ on my desktop fine, Steam Deck gets 12.5 MB/s. I’ve tried with the Steam Deck dock, I’ve tried with a USB-C Ethernet dongle, and it stays stuck there no matter what I do.
Hey so I just found something that fixed my issue. To test it, you’ll have to go into Desktop mode. Have to open the Steam Console, and the instructions in this thread don’t work for it. What I had to do was to go to the steam deck icon, click it, type “Konsole” in the search bar, open Konsole, then type the following:
Yes, you type steam twice. Then follow the instructions to paste the two lines in individually. I did that and my speeds instantly shot up to 50 MB/s from 12.5 MB/s
Hopefully this helps you too, as your issue sounds similar to mine.