Updated FAQ (markdown)

FAQ.md

@@ -393,10 +393,23 @@ Ensure your relay server (`hbbr` which uses TCP `21117` port) is running on the
 ![image](https://github.com/user-attachments/assets/90c51c37-8ec4-4bcf-96e3-e8d21b60c878)
 
 - Case 1
+
+> User: All other devices can connect and work normally, except for one specific device. However, this device does show as online.
+>
+> RustDesk: Please try `telnet server-ip 21116` and `telnet server-ip 21117` on the controlled side.
+>
+> User: Both telnet worked, but I tried to connect from the problematic device to other devices and encountered error: ***invalid public key***
+>
+> RustDesk: That's the problem. Please set correct public key on the controlled client. https://github.com/rustdesk/rustdesk/wiki/FAQ#key-mismatch
+
+
+
+- Case 2
 > User: We are currently experiencing major problems connecting to a large number of clients. Some connections work without problems, but for most, the connection window "Connecting" appears, the circle spins, and then nothing else happens.
 > 
 > User: Never mind, the relay server wasn't listed after the update.
 
+
 We run the server on a Synology Docker container.
 
 # The connection is interrupted every 10-15 seconds
@@ -510,7 +523,7 @@ The controlled side works fine with UDP `21116` port, but it can not connect to
 
 # Failed to secure tcp:Handshake failed: invalid public key from rendezvous server: Please try later 
 
-Please check `Setting` -> `Network` -> `Id/relay server` on rustdesk client, and the the screenshot of `Settigns` -> `Key` on the web coonsole. Need to confirm they have the same key (public key).
+Please check `Setting` -> `Network` -> `Id/relay server` on rustdesk client, and the the screenshot of `Settigns` -> `Key` on the web console. Need to confirm they have the same key (public key). 
 
 # Deploy RustDesk server in intranet.