iPhone 11 Pro Max connects, then drops, primary network - guest seems ok

I'm running an RBR850 and two RBS850s. Things have been great for a while. Yesterday I started experiencing problems with my iPhone 11 Pro Max staying connected to the primary network. It would be at 600+Mbps/40Mbps download/upload, then it would suddenly drop to maybe 1/40, or so low on the download side that I could not even run SpeedTest. The network was fine, as all of my other devices - including other iPhones and several Macs - worked fine. Eventually I figured out the Guest Network was an option (slower) to the iPhone 11, but I don't understand (a) what happened so suddenly (as I didn't do anything differently with the Orbi setup), and (b) why the guest is easier to stay connected - as they both have the same security setup.

Any recommendations? I've already gone through all of the iPhone recommendations - including the sucky step of a complete factory reset. As of this morning that seems to have fixed it (this is the second time I've had to do that, so perhaps it's an Apple problem - but still I don't understand the different results).