Can you try a configuration like this:
radius-server host 192.168.55.57 key "secret"
aaa authentication port-access eap-radius
radius-server host 192.168.55.57 dyn-authorization
radius-server host 192.168.55.57 time-window 0
aaa port-access web-based 1-2
aaa port-access web-based ewa-server www.example.com
aaa port-access web-based 1-2 redirect-url www.google.com
The redirect-url is a page that you want to re-direct them to after authentication. the ewa-server will specify the webpage to use for login. Also make sure captive portal is enabled on the switch as well.