docs(fqa): add faq in user guide (#234)

Co-authored-by: sibianl <sibianl@rings.net.cn>

Author: sl-gn

docs/user_guide/quick_start.md

@@ -169,3 +169,18 @@ curl --location 'http://localhost:3000/v1/chat/completions' --header 'Content-Ty
 }'
 
 ```
+
+### FAQ
+> Q: When deploying on cloud servers, I encounter an error like "lattica RPC call failed". What does this mean and how can I resolve it?
+
+A: This error typically occurs when the necessary network ports for communication between the scheduler and nodes are blocked—most often due to firewall or security group settings on your cloud platform.
+
+**How to fix:**
+- Ensure that the relevant TCP/UDP ports for both the scheduler and nodes are open and accessible between all machines in your cluster.
+    - By default, the scheduler uses HTTP port `3001`, and nodes use HTTP port `3000`. You can change these with the `--port` argument (e.g., `parallax run --port <your_port>` or `parallax join --port <your_port>`).
+    - For Lattica (node-to-node) communication, random ports are used by default. It is best to explicitly specify which TCP and UDP ports to use (e.g., `--tcp-port <your_tcp_port> --udp-port <your_udp_port>`), and then open those ports for inbound and outbound traffic in your cloud provider's security settings.
+- Check your cloud provider's firewall or network security group configurations:
+    1. Open inbound rules for the ports mentioned above on all scheduler and node machines.
+    2. Make sure that ports are open to the desired sources (e.g., to all cluster instances, or to your public IPs if required).
+
+After updating the firewall/security group settings to allow these ports, restart your scheduler and nodes.