IOS Update through tethering I am not having WiFi. To update to IOS 11 iphone needs a WiFi connection. 
My idea is to use the Iphone for tethering and connect it "to itself". Will it work? Can I connect my Iphone the WiFi it makes itself and than do the update via mobile network?
No. 

It’s not possible to connect to an iPhone to its own Personal Hotspot network as a client. iPhone can only be connected to one Wi-Fi network, so any attempt to join a network will turn off Personal Hotspot.

You can tether to any other device, though and get your download on cellular data from the other plan.