The technology is called "
Captive portal".
You did not mention on what basis (what solution) Your server gives out wifi, but here's an example of iptables:
https://habrahabr.ru/sandbox/79059/
Another review of the decisions on the subject:
https://xakep.ru/2013/10/29/captive-portal/
If they don't fit, now knowing the name of the topic, you can look for a solution that fits Your mechanism of distribution of Wi-Fi.
Do fine - would be fine. (C) - Carli commented on July 9th 19 at 10:57