Trouble with hotspot

Good afternoon people. I have a doubt and would like to help. I have a client who needs the following situation. I have a central radius, and when the customer enters his restaurant and try to navigate, I would like him to be redirected to a site .. example: site.com.br/login it and he should make an login or register and then was released to navigate.
However, it takes place that will not have access to the modem, so I can not rely on public IP to access and send commands to mikrotik.
Is there another way to do this?