Arma 3 server status checker
Arma servers listen across a small run of consecutive UDP ports, and the query answer comes from a different one than the game itself.
Which ports a Arma 3 server needs
The game port is what players enter, and the query is answered on a port immediately above it. Forwarding only the game port produces a joinable server that no list and no checker can see. Arma also uses further ports in the same range for other services, so a range rule is safer than individual entries.
If it still shows offline
Open the range rather than the single port and retest. If the query still fails, check that the server finished loading its mission and mod set — a heavy modded server can take a long time to become queryable, and BattlEye initialisation adds to that.