Heads up for those who run a dedicated server who haven’t tried experimental first, posting this so it’s on your radar.

Long story short, we had to open two additional ports on our server, 8888 and 27777. Following is a direct copy from the changelog:

Dedicated Server - Port Forwarding Updates

If you have Host a dedicated server, you should definitely give the next block a read as many new improvements have been added to allow for extra flexibility since the last update on Experimental

We have updated the Port Allocation Strategy in Reliable Messaging New features:

Explicit Port Configuration

• A new -ReliablePort= command-line parameter allows explicit port selection.
• The value must be an integer between 0 and 65535.
• If specified, the server will attempt to bind to this port and fail to initialize if the port is unavailable.

Default and Configurable Port Ranges The following settings in Engine.ini control port allocation:

[/Script/ReliableMessaging.ReliableMessagingTCPFactory]
PortRangeBegin=8888
PortRangeLength=512
ExternalPortRangeBegin=-1

• The server will attempt to bind within [PortRangeBegin, PortRangeBegin + PortRangeLength).
• By default, the server starts at port 8888 and tries up to 512 ports until it finds an available one.

Client Awareness & NAT Handling

• Clients must connect to the correct port, but port remapping (e.g., via NAT/firewall rules) can break this.
• To address this, the server now communicates the listening port to clients during the initial handshake.
• If external port remapping is used, the server must be aware of the external port via:
• The ExternalPortRangeBegin config setting (for remapped ranges).
• The -ExternalReliablePort= command-line parameter (for explicitly mapped ports).

Server Host Requirements (TL;DR)

• If hosting a single server, port 8888 TCP must be open by default.
• If hosting multiple servers, a range of ports starting from 8888 TCP (by default) must be open.
• The server will attempt up to 512 ports before failing (configurable).
• If port remapping (NAT/firewall) is used, the server must be configured accordingly; otherwise, clients won’t be able to connect.
• Logging is in place to help server maintainers verify the allocated ports.