While I was able to use my iPhone as internet hotspot when local (in residence) cable internet via AirPort
wi-fi base, failed; during troubleshooting and through a few days until acquired replacement hardware...
the setup and use of Hotspot was most easy compared to actual configuration of replacement wi-fi base.
So due to system vintage and associated details, the situation you've related in the reply does match any
in my limited experience. ~ I've mostly used Macs, from local wi-fi; or cable internet, &/or DSL modem.
{..And before those, was on super-slow dialup internet; between 'glacially slow' to 'calving iceberg' slow.
Plus iPhone (local wi-fi) & iPad Mini; along with most MacOS from early classic 6.0 through OS X 10.11.6...}
Given the details & options within the links already provided, I was able to troubleshoot/resolve several issues.
But these things can vary due to extenuating circumstances; some outside our control. ~ You may have
other settings on/off that disallow your connection from taking place. -- Or other (?) hardware issues.
Have you tried resetting/restarting the iPhone? An item in 'settings menu' allows you that as an option.
Or holding buttons, depending on your iPhone model & iOS version.
Sorry to not have exact match..! 🎣
PS: can Anything from here help?
If Personal Hotspot is not working on your iPhone or iPad
https://support.apple.com/en-us/HT203302
On the iPhone or iPad that provides Personal Hotspot,
go to Settings > General > Reset, then tap Reset Network
Settings. If you still need help, use the steps below for help
with Personal Hotspot over Wi-Fi, USB, or Bluetooth.
(this may just be a repeat from another search!)
The other direction suggests these ideas:
How to Turn Your Mac Into a Wi-Fi Hotspot
https://www.howtogeek.com/214053/HOW-TO-TURN-YOUR-MAC-INTO-A-WI-FI-HOTSPOT/
This will create a Wi-Fi hotspot, and the Internet connection you selected at the top of the window will be shared
with devices that connect to the Wi-Fi network. Click the "Wi-Fi Options" button at the bottom of the window to
configure your Wi-Fi hotspot. Select your preferred network name and the best Wi-Fi channel.
