If I turn off every wifi device on my network, hardcode my IP and DNS, then turn off DHCP on my router, it solves the issue. This is not an answer though, as I have about 28 Wifi devices, and some are difficult to hardcode. I cannot disable DNS on my router, only by disabling DHCP can that be done.
The only way I can do an update (like from 13.5.1 to 13.6 beta) on Wifi is to connect to another wifi router, where I am the only device.
This was definitely introduced with 13.5.1 as that was when my issues started. My Photostream stopped working at that time also - presumably because it thinks all my iDevices are on cellular.
I've reported the bug. I can't pin down how to fix this - I'm pretty technical, but if Apple is broadcasting something on the Wifi that's interfering, then I'm at a loss. One theory is something is triggering devices to think they are on a Personal Hotspot, as they behave the same when actually connected to one.
App store cannot be accessed anymore using wifi - even my macbook was affected. Using mobile data seems to work. But had to edit some settings in my macbook certificate so I could access app store via wifi.
Some apps become offline when on wifi. I could not use apps like tinder when on wifi. Mobile still is working