The captive portal solution i know capture the first http request and redirect the browser to the login page. Firefox uses this URL to detect a captive portal: http://detectportal.firefox.com/success.txt
You can test this directly after your wifi connect with: curl -v http://detectportal.firefox.com/success.txt
You should see a 302 redirect.
What is OpenWISP?
OpenWISP is a software platform designed to ease and automate the management of networks, with a special focus on wireless networks, mainly used in public wifi, mesh networks, community networks and IoT scenarios.
OpenWISP 2, launched in December 2016, is the new generation of the software which is gradually ...
Another option is that the MacOS likes to scan for available networks periodically. To do this, there is a brief disconnection from your WiFi. There is a setting in the Mac to connect automatically to nearby networks and this can be turned off. There is also a wifi configuration (I can't recall it) to keep it from trying to jump from AP to AP frequently. ...