I thought it might be helpful to others to share my experience with trying to get crossplay to work on the steam deck.
After trying for 4 days to get a server started just through valheim so that I could port items and stuff to my new Xbox character from my steam deck fella, I finally bought a hosting service subscription. Nothing I could do would allow the systems to play with one another online, either with the Xbox as the host or the steam deck.
Then, after not being able to get it to work with the hosting service, I spent about an hour with their tech support and found that I steam and Xbox were able to play together if the steam player was on laptop. No matter what we tried (and we tried a LOT of things) we could not get the steam deck player to join the dedicated private server. Xbox player had no problem, but the deck inexplicably could not join the hosted game.
It’s taken me 5 days to get to this point, if not more. Hopefully this can save some others some time if they are experiencing the same issues with the steam deck.
Source: https://www.reddit.com/r/valheim/comments/11wt9od/psa_for_steam_deck_users/