I ran into similar issues after the most recent firmware update to my Marantz SR6008. Control through the Denon/Marantz app was failing as well. I contacted Marantz and they sent me information on how to perform a network reset. It involved a button sequence on the unit, not just the standard unplug/replug cycle. Since then everything has been fine.
Denon Avr E300 Firmware Update
Download: https://urluss.com/2vKkkj
I had noticed the Denon failed on the firmware upgrade so the local Rep told me to bring the unit back to exchange for a new one hoping this was the problem. Well, the firmware update on the new receiver completed fine, but I actually had more problems with airplay, it's working now, but still the same songs won't play.
The lack of Wi-Fi is more understandable, but it still means you need to have Ethernet in your living room (or use a work-around) to take advantage of the AVR-E400's networking features, such as AirPlay, DLNA, smartphone control, firmware updates, and streaming services such as Spotify, Pandora, SiriusXM, Flickr, and Internet radio. We toyed around with the AVR-E400's streaming features a bit, and while they'll suffice for basic use, anyone who does a lot of streaming will want a dedicated device, mostly for faster navigation. 2ff7e9595c