Rust+ is not a feature of your game client. It is a feature the server exposes. When the in-game Rust+ panel says Rust+ is disabled, or the Pair with server button is missing, the server has Rust+ off. That happens in two ways: the owner disabled it, or the server's companion port is not reachable from the internet and Facepunch's automatic connectivity test disabled it. There is no phone setting, reinstall or account fix that changes either.

Confirm it in 30 seconds

1. Stay connected to the server in question. Press ESC and open Rust+. A working server shows a Pair with server button. A server with Rust+ off shows a notice that Rust+ is disabled instead; Facepunch made this visible on purpose so players stop blaming the app.
2. To prove it is the server and not you, join any official Facepunch server and repeat. Official servers show the button. If you see it there and not on the community server, you have your answer.

Why servers end up with it off

On purpose. Rust+ lets players see team positions and control smart devices remotely, which some hardcore and roleplay servers consider unfair. The proper way to disable it is app.port -1, and the in-game panel then says so.

By the connectivity test. This is the common accidental case. When the server starts, it checks that its own companion port can be reached from the internet. If the check fails, the server logs Rust+ companion server connectivity test failed! Disabling Rust+ features. and turns Rust+ off until the next restart. Typical triggers: the host never opened the companion port, the port is below 10000 (Facepunch's backend refuses those), the server sits behind NAT without app.publicip set, or a firewall change after an update. The admin often does not know it happened.

What the admin has to do

The server needs four things, all on the server side. If you know the admin, send them this list.

1. Set the companion port. The convar is app.port. It defaults to the game port + 67 or the RCON port + 67, whichever is larger (28082 on a stock setup), and must be 10000 or higher. -1 disables Rust+ entirely. app.info in the console prints the port and the IP the app will be told to connect to; if that IP is wrong (NAT, multiple interfaces), set app.publicip, and app.listenip if the server must bind a specific interface.
2. Make that TCP port reachable from the internet. Forward it on the router or open it in the host's firewall, exactly like the game port. The phone app connects to it directly, and the server's own connectivity test does too. Some panels expose this as a "Rust+" or "Companion port" toggle; others need a support ticket. To test from anywhere: open a WebSocket to ws://<server ip>:<companion port> (Postman can); if it connects, the port is open.
3. Let the server reach companion-rust.facepunch.com. Outbound access is needed to register the server and send pairing notifications.
4. Restart and read the log. Port changes need a restart. If the log still shows the connectivity-test failure, step 2 is not done yet.

The one case where it is your side

If a teammate on the same server sees the Pair with server button and you do not, at the same moment, the difference is on your client, not the server: disconnect, reconnect and open the Rust+ panel again. The panel reflects the server's state at the time you joined, so a server that re-enabled Rust+ after a restart shows correctly only after you rejoin.

Not the same as "Pair does nothing"

If the button is there but pressing it produces nothing on your phone, the server has Rust+ on and the fault is elsewhere. Continue with Rust+ won't pair or gets stuck.

For rustplusplus users

The same rule applies to the bot: it cannot pair with a server that has Rust+ disabled, because no pairing notification is ever sent. If your bot lists some servers but never a particular one, check that server's ESC menu before touching the bot or your credentials.