You would set up a routed client. The easiest way to do this is to use the existing wan firewall zone. I think the RE450 is a single Ethernet port device, so there is likely not a wan network pre-defined, but you can create one and attach your wifi client to it.
The public network only sees the MAC of your router. Use any device on the LAN to log in the captive portal then the router is authorized and you can use multiple devices on the LAN with the same authorization. You would only need to change the MAC if you have some permanent authorization registered for one MAC.