FWIW to anyone else (and I can't believe Roku hasn't resolved this issue in several months!): Through trial, error, and luck I found a fix. I'm using a Roku HDMI Stick and an iPhone 6s. Hotspot's name is very short and neither it nor its password use special characters or spaces. Tried rebooting both more times than I want to admit. Roku could see the hotspot but threw error 14.40 when trying to connect. The trick was to keep the iPhone displaying the hotspot config screen (the one that shows your hotspot password) through the ENTIRE connection process. I only thought of this test because my daughter's unbelievably antique iPad requires the same thing. Everything works, and I love my Roku, but shame on them for not paying more attention to this basic issue.
Your solution was with a 6S. I never had any issue using my 6S when connecting my Ultra or a Roku TV to the hotspot. These people are specifically having an issue with the latest version of the iPhone, which seems to do something different from earlier phones. It's great you found a solution that works for you, but I have my doubts it will help the others. But I'd be happy to learn otherwise
Nvidia Shield, Roku Stick (3600), Ultra (4640), Premiere (3920), Insignia 720p Roku TV, Sharp 4K Roku TV, Windows 10 Pro x64 running Serviio and Plex on a wired Gigabit network.