I've bootcamped my Macbook (I can run OSX & W10).
When I boot up OSX, tether to the phone, setup the VPN connection within OSX (using Cisco IPSec), I can log into the VPN no problem, it's instant.
When I boot up W10, tether to the phone and try to setup the VPN connection using any configuration of the W10 VPN client, I just can't get it to connect.
I don't think it's the phone either, I think it's the W10 VPN client in some way.
I've looked for alternative VPN client software for Windows 10, but not been able to locate any.
So I'm wondering if anyone has had success running IPSec via the W10 VPN client through an iPhone with iOS 12+???