1. The login token
The server log says "Steam Game Server Login Token (GSLT) not set" when it has none, and the server is then hidden and cannot be joined over the Internet. Make a token for app 304930 and set it. If the log says Steam refused the token, make a new one.
2. Is it still starting?
A first start downloads about 2 GB, and then any Workshop maps. The server is listed only after "Loading level: 100%".
3. Try the Server Code
Connect Directly with the code from the dashboard. If that works, the server is up and the rest is about the list.
4. The exact name and the filters
Search the Internet list for the exact name. Filters for map, password, PvE or perspective hide servers that do not match them.
5. A Workshop map that updated
When the Workshop map it runs is updated, the server warns players and shuts down to update; it is gone from the list until it is back up, a few minutes later.
Frequently asked questions
Why is my Unturned server not in the server list?
Almost always a missing login token: Unturned hides servers without one. Set a token for app 304930.
Can people join without a token?
Only with the Server Code, which changes on every restart. Over the Internet by address, no.