I have a GL-750S-Ext Slate running firmware 3.203 and did not have any problems with setup. I have it as a router with wireless WAN connected to another wifi for Internet and wired/wireless LAN for connection to Windows laptop, Android smartphone, Chromebook. OpenVPN and Wireguard both work also,.
I think the GL-750 Creta would be similar. What specific problem(s) did you encounter with the Slate? The GL.iNet online guides are good and should be followed carefully.
I have not tested going through a captive portal yet because I have not been travelling for a while. There is a tutorial for captive portals: