Another thought:
Note: If you see an "Unable to Join" or timeout error when trying to connect a device using Wi-Fi, ensure that you remain on the Personal Hotspot screen until the first device connects. After a device has connected, you can leave the Personal Hotspot screen and use your phone normally. Your device will automatically stop broadcasting using Wi-Fi if you put your device to sleep or after 90 seconds elapse with no devices connected using Wi-Fi. You can start broadcasting your Wi-Fi network again by tapping Settings > Personal Hotspot.
(Apple Support)
This is the same if you use another phone (not iPhone) as hotspot. After 90 seconds you won't get a connection.
If this doesn't work, you should do as MoonlitSonata suggested.