The preferred way to connect on these “guest” networks is to leave the DNS entry blank. When you connect to the service the internal network server will supply the DNS that allows your computer to find the web page, where you can accept the terms.
This DNS snafu may also apply on a guest network at a hotel or coffee shop. On My Mac, I made a new location called “ViaRail WiFi” and left the DNS empty. When I get back to my home/office network, I will switch it back to my own preferred settings.