Here is the one I was thinking of.
I don't think that LuCI or uci support this mode of operation. You will have to do it with the CLI and scripts.
The post describes how to create a wpa supplicant config file with your credentials then launch an instance of wpa_supplicant in wired mode from the CLI. The command points wpa_supplicant directly at an Ethernet port. Once authentication is completed, then you use regular network setup (typically DHCP client) to configure access of the network.
The example is a simple TTLS / PAP user-password scenario. If your site requires you to present a client certificate or you want to validate the server certificate, you will need to read the main wpa_supplicant documentation to include them in your config file.