hello @Gibson , it will depend on the reference used exactly, there are several ways to do it, the easiest one https://kolibri.readthedocs.io/en/latest/install/raspberry_pi.html does not have a nginx captive portal so I am a bit lost.
If you did it using kolibri-server
you can do dpkg-reconfigure kolibri-server
and it will show you a list of ports to run kolibri and will update the configurations according to your selection.