DayZ server status checker
DayZ separates its game port from its Steam query port, and the query port is the one that decides whether your server is visible.
Which ports a DayZ server needs
Both are UDP and the numbers are unrelated. The query port is in the Steam range rather than next to the game port. A server with only the game port open works for anyone who knows the address and is invisible to everyone else.
If it still shows offline
If both are forwarded and the check still fails, confirm the server has finished its startup cycle — DayZ loads the central economy and spawns the loot map before answering, which takes a while. Modded servers extend that considerably.