Mobile devices tend to not like being on an air-gapped network (i.e. not connected to the internet) because they will usually switch to their cellular connections to maintain connectivity. However, they will usually give the option to stay on wifi despite the lack of internet connectivity.
You can try the "fake internet service" package...
That said, it would be good if we understood your desired use-case. As pointed out previously, a captive portal doesn't really make a lot of sense when internet connectivity is not available, but there may still be reasonable applications for it and/or other methods to achieve your goals.