I have tracked down my issue to being an incompatibility between iOS 13.5.1 and above and a fairly new DLink router. There was no issue until I updated to 13.5.1. Not sure if it's a DLink issue or an IOS issue, but it's definitely the DLink combined with iOS 13.5.1 and up.

I'm having similar issues, but with 13.5.1 and 13.6 Beta 2. My issue is a bit different - my devices technically work, but they all believe they are on cellular. If I try and do an IOS update, it hangs on Update Requested, and something about my network not being suitable. If I try to download large app (even on an iPad Wifi-only device) it asks if I'm sure I want to download on cellular. If I say to continue it downloads on WiFi. 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.

Apparently you need either Big Sur, or download the beta version of Xcode and it will install it.