Choosing hotspot automatically switches to WiFi.

My internet provider provides fast but not 100% reliable internet. Up to a couple of years ago I solved internet outages by switching off WiFi on my iPhone and then switching off WiFi on my MacBook Pro and switching to my iPhone hotspot. This worked and I even have unlimited internet access on my phone contract to cope with these outages. Since a couple of years ago this no longer works as the MacBook Pro switches automatically back to my WiFi which is still working but connected to a dead internet. Is there a way to stop my MacBook switching back to WiFi?