To date there is no option in the menus to specify the address of a server to connect to when playing with the epic game version.
Here is the way to proceed while waiting for the game developers to react:
You will have to start a game in single player mode then open the console (tab key by default) and type the following command:
open ipaddress:port
by replacing ipaddress:port with the ip address and the port of your RoxServers.com server
For example if your server has for address 123.456.789.0 and for port 12345 then you will have to type:
open 123.456.789.0:12345
Tip: on your RoxServers.com control panel you will find this address and port at the top of your server management page as follows:
Connection Info: 123.456.789.0:12345