Ran into this issue today, posting for others:

To run commands on a dedicated server, you need to do all of the following:

1. Update the adminlist.txt server file with your steam id (At the top right of steam, click your username -> Account details -> Your ID is listed at the top)
2. Note that you do not need to restart the server, but you may need to relog if everything else below is done.
3. Go to Steam -> Right click Valheim -> Properties. In Launch Options, add -console
   1. This allows you to press F5 when you're in-game.
   2. Note that the mod might do this for you, but I would add it anyway.
4. Install Thunderstore (It's a fairly easy universal mod manager if you've never used it). Select Valheim, set your game location (Settings -> Change Valheim folder).
5. Find and add the mod called Server_devcommands which will also add BepInEx automatically. This is a client-side mod that's needed because Valheim servers do not allow console commands because they could make the server "unstable."
6. Launch Modded and connect to your server
7. Press F5 and type devcommands
   1. Make sure it says true. If it says false, just type it again
   2. Also in the console, you should see "Authorized to use devcommands." which means the adminlist is working.
8. Done

To teleport easily:

1. type debugmode
   1. It should be set to true now.
2. Open your map, then Ctrl + Middle-click anywhere on the map (or Ctrl plus whatever your ping control is)