(for that second page, see the table, specifically "RHQ Server Hostname or IP Address").
In short, make sure the server's public endpoint address as defined in the GUI is resolvable by the agent.
When the agent starts, it talks to the "RHQ Server Hostname or IP Address" you gave it at startup/setup time. But the agent will only use that hostname/IP to register that one time. Once registered, the server will tell the agent what server(s) it should talk to from here on out. And those hostnames/IPs come from the server - as listed in the Administration>High Availability>Servers page.
THANKS! Look, it's in the faq, even. Silly me. Kinda confusing, tho.