One of the first recommendations on the Deck is to disable wifi power management:
If you’re having connection issues on the Steam Deck, especially on a 5Ghz connection, you might need to disable “Wifi Power Management.” Disabling wifi power management may increase connection reliability on 5Ghz networks. In order to access this option, you need to turn on developer mode...
tuxexplorer.com
I had similar issues, it would drop and reconnect all the time until I disabled power management (even sat right next to my wifi AP).
Ive tried that, unsure at this stage if it has improved it, i havent seen the exclamation mark near the wifi symbol since, but the connection still seems dreadful.
I was downloading a game last night, the download speeds were all over the place. in the end it went down to zero, while still "connected" and it just sat there doing nothing. i eventually switched it off in frustration and went to bed.
I use a Virginmedia hub thing, the previous version of which caused issues with some old amazon tablets the kids had. the new version seems ok, but i would no doubt benefit of using it in modem only mode and getting a better wifi router. i'll be annoyed if i shell out for a new router and its still the same though.
EDIT, so now its on the office wifi, it carried on downloading the game it was doing yesterday, although its lost 5 gigs of it somehow and its now back at 50 something percent when it was 90 something yesterday. It started out strong, sat at a consistent 2.8 MB/s... then i looked at it again and its sat there a 0 B/s and has done since. There was a few seconds of activity again, and now its back at 0.
Its writing to an SD card, so i guess it could be that... i used the card in my old phone with no issues though. i could try downloading to the internal storage and see if that is any better. Its done absolutely nothing for the last ten minutes.