When I try to log in for the simulator in iOS 14, the loading spinner just spins forever. There is a thread for this on the Apple developer forums here but no solutions and no response from Apple. I can certainly test on the device, but testing on the simulator is obviously easier for iteration. Does anyone have a solution to this problem?
You can download the iOS 13 simulators to test SIWA on the simulator or, as you already said, you can test on iOS 14 on a device.
I would also suggest that you send a bug report to Apple. The more bug reports they receive the more likely it is that they will fix it.