Hi heywood.
When the client starts the first time, it selects a free port in a range defined in settings and tries to always use the same, in order to ease the firewall configuration and match the UDP rules.
When there are multiple instances running, each client using UDP selects a new port in the range instead of the “default” port. You can change both the default port and the range in Edit => Advanced => UDP settings (visible only if NX protocol is selected).
You should only need to configure your router to open the range of selected ports.