I can't turn on iPhone Hotspot with my home WiFi turned on. Why? I want to share my 4G connection with my laptop since our WiFi connection at home can be slow. However, once I go to the iPhone Settings > Personal Hotspot and enable the hotspot,  the iPhone asks me to enable its WiFi antenna. 

At this stage, the iPhone automatically connects to my WiFi network at home and I cannot really transfer the 4G connection to the Hotspot. 

What should I do (apart from connecting the iPhone via USB to the laptop)?

More importantly, why do I loose the 4G connection and connect to the WiFi network instead of the iPhone being smarter and procuring my 4G connection? 

This has happened in the past very rarely and I could always enable 4G iPhone hotspot over WiFi. Now the problem happens each time I accept the prompt to connect to WiFi.
Try turning off your home WiFi. A kind of interference, maybe?