"Rust+ isn't working" is five different problems that look alike from the phone. The fastest way through is to name the exact symptom first, because each one has a different cause and most fixes for one do nothing for the others.

Find your symptom

What you see	Where the problem usually is	Fix
The in-game Rust+ menu says Rust+ is disabled, or there is no Pair with server button	The server has Rust+ off, or its companion port failed Facepunch's connectivity test and Rust+ switched itself off	Pair button missing
You press Pair, nothing arrives on the phone, or it sits on "Pairing…" forever	Pairing notification never reached the phone, or the server can't reach Facepunch	Won't pair or stuck on pairing
Server is paired but shows Offline while you are playing on it	Your phone can't open a direct connection to the server's companion port	Server offline
Pairing works, but smart alarms, deaths or team logins never notify you	Push delivery on the phone, or notification opt-ins in game	No notifications
The app keeps saying Please sign in to the Rust+ app	Steam sign-in state on the phone, not the server	Sign-in loop
Failed to set up notifications or Failed to get information from server right after pairing	The handshake after pairing: phone → server, or phone → Facepunch	Post-pairing errors
The app's server list is empty, or your server is not in it	Pairings belong to an account and a device; something reset them	No servers in the app
A smart alarm shows inactive, never fires, or fired once and went quiet	Wiring order, pairing, or the cooldown	Smart alarm
Camera feed is grey and blocky, or says No Signal	That is how the feed works; No Signal is power, identifier or authorisation	Cameras
You are on Xbox or PlayStation	Rust+ is PC-only	Rust+ on console

If two rows match, fix them top to bottom: pairing has to work before anything below it can.

How Rust+ actually connects (60-second version)

Knowing this turns guessing into diagnosis.

1. Pairing is a push notification. When you press Pair with server in game, the server asks Facepunch's companion service to send a notification to every phone signed in with your Steam account. It rides Google's push system (FCM) to your phone. If the server can't reach Facepunch, or your phone can't receive pushes, pairing fails even though the game itself is fine.
2. Live data is a direct connection. Once paired, the app talks straight to your Rust server over TCP on the server's companion port (by default the game port + 67 or the RCON port + 67, whichever is larger; 28082 on a stock setup). Map, team positions, server info, smart switches: all of it flows over that socket. "Server offline" in the app means that socket didn't open.
3. Alerts are pushes again. Smart alarm triggers, offline deaths, teammate logins and so on go server → Facepunch → FCM → phone.

So there are three independent links: server ↔ Facepunch, Facepunch ↔ phone, and phone ↔ server. Each symptom above breaks a different one.

Two checks that settle most cases in under a minute

Check 1: try an official Facepunch server. Official servers have Rust+ enabled and correctly configured. Join one, press ESC → Rust+ → Pair. If pairing works there, your phone and account are fine and the problem is the community server you came from. If it fails there too, the problem is on your phone or account.

Check 2: make sure you are actually connected to the server when you press Pair. Rust+ pairs the server you are currently playing on, not one from the history list. Pressing the button from the main menu after disconnecting sends nothing.

What server admins control and you can't

Rust+ is opt-in per server. The owner decides whether it is enabled (app.port set to a port of 10000 or higher instead of -1), whether that port is open through their firewall and host, and whether the machine can reach companion-rust.facepunch.com. Since Facepunch added an automatic connectivity test, a server whose companion port is not reachable from the internet turns Rust+ off by itself and logs Rust+ companion server connectivity test failed! Disabling Rust+ features. If the official-server test passes and the community server still fails, there is nothing left on your side; send the admin to Facepunch's Rust+ Server page. Most hosting panels have a Rust+ toggle.

Running Rust+ through a Discord bot instead?

If you use rustplusplus rather than the phone app, the same three links apply, with your bot standing in for the phone. The pairing notification is delivered to the bot through the FCM credentials you generated on this site, so a bot that never receives pairings almost always has expired or missing credentials (they last 14 days). Generate a fresh set with the Log In button, run the /credentials add command it gives you, and pair again.