Well, one thing I noticed with my Samsung hotspot, as shared by Samsung phone...

Is that if the Samsung phone is NOT plugged into a power-charger, then I get all kinds of spotty weird results, including slow downs, and time-outs, and "Internet-not-available" type of warning messages.

So... Try this:

COMPLETELY shut down your PC.

Next: COMPLETELY shut down your phone.

And then: Start up your i-phone.

And: plug your i-phone into a good power charger.

And then, finally: start up your PC, and log into the hot spot.

Hopefully that solves the problem. If it does, then it means you have to have your iPhone plugged into a power supply, to handle the demands of acting as a router-hot-spot service provider.

But in the end...

Perhaps that doesn't help?

If not, do you have a friend that:

1) uses their iPhone,

2) and their PC computer,

3) with the same service provider as you use?

If yes, then your service provider will need to help you through this.

If after a lot of time with your service provider, you can't solve the problem, then consider hiring a networking experiment (about $50 to $100 per hour, or a flat fee to solve the problem for you).