There seems to be an issue with third party keyboards on iOS when being prompted to enter WiFi access data after selecting a network. I found potential solutions on Apple’s support communities, where the suggestion was to reset network settings, but it was not working for me. Hmm.
Anyways, quite by coincidence I found out it was my SwiftKey keyboard which was blocking or preventing me from entering user credentials. Actually, you should disable or uninstall the keyboard in order to be able to enter user credentials. But, what solves this issue as well is going into any other application, where the keyboard opens up (like notes or any messenger), switch the keyboard from within there, and go back to the WiFi settings.