[faq] Mention inbound UDP 51820

Author: Silex

src/pages/about-netbird/faq.mdx

@@ -6,6 +6,7 @@ export const title = 'FAQ'
 
 ### Incoming ports
 NetBird's agent doesn't require any incoming port to be open; It negotiates the connection with the support of the signal and relay services.
+That said, it helps to open inbound UDP to port 51820 on one of the peers if they are only getting relayed.
 ### Outgoing ports
 NetBird usually won't need open ports, but sometimes you or your IT team needs to secure and verify
 all outgoing traffic, and that may affect how NetBird clients connect to the [control layer](/about-netbird/how-netbird-works)