As expected. Problem is that going on-site is not always wanted, and just clicking that Agree button is some interactive thing. People good in scripting, could probably script that action. (Is what they have done in OpenWRT Travelmate)
If not , then you need either a local or remote browser connection. This is possible and quite easy through a SSH tunnel, but unfortunately the Goodcloud SSH does not support a tunnel to be used by the browser.
The Goodcloud SSH and GUI interface could be used to set up an ad hoc VPN tunnel which would allow this browser access to the portal. Not worked out today, so today I do use a VPN to another AP (with a build in web-proxy server) that AP is connected as client to the GL.inet router via its client wifi. Many other combinations are possible (e.g. remote screen to a PC).
We do have SSH through Goodcloud, so some extra software could allow a tunnel in that tunnel. Have been reading about a "poor man" tunnel SSHUTTLE (requires Python), what is supposed to work here over any SSH connection. The other thing would be to add one of those travel routers with Zerotier, or any other VPN or remote access to a client device. (Teams, GoToMyPc, Zoom, Teamviewer, VNC, .... and many others)
Also check this about TravelMate : Automatic sign-in to free wifi with GL-MT300N-V2? - #6 by buntspext
EDIT
More options to get interactive browser access to that portal page ...