Wanted to post my Airplay connection problem and solution in case anyone has a similar setup to mine and is still trying to figure it out.
Denon 2112ci (newest firmware, just bought this a week ago)
Linksys E3000 router
iMac, iPhone, Android phone
Synology Diskstation 212 Network Attached Server
Denon is hooked up to ethernet via DLink Gb Switcher, which is accessing router via Netgear MoCA Coax Ethernet Adapter
After setup of Denon, everything worked fine including internet radio and streaming music from Synology through the Denon via Android phone (using Synology app) or via Net/USB input.
Only problem was Airplay icon would not show up in iTunes or iPhone. Not end of the world since I can just play my music files via Android phone from Synology, but regardless I have to have everything working since Airplay is what the receiver is supposed to be able to do.
Tried everything suggested in different forums but could not get Airplay to show up, certain it was an issue with the router setup. I read many posts where people changed to a new router and that solved the problem, but I was sure it was just a setting.
On Cisco forum, found a post for my router of where to enable "multicast" on router admin page (security tab, firewall). I guess this allows Apple's Bonjour to forward. But first had to update firmware. Did both, but still no Airplay icon. Unplugged Denon, unplugged DLink ethernet switcher, plugged both back in, and NOW I HAVE THE AIRPLAY ICON!
Been playing through Airplay for over an hour, and have had steady connection, no hiccups until someone called my landline cordless phone (which is on 2.4 GHz) which then interrupted the wifi signal and stopped the music. Got the telemarketer off the phone, checked the iPhone and Airplay icon came back, started up music again, running fine for couple hours now.
I'm hoping the reliable connection continues and I have no issues down the road.
So before thinking you need a new router, be sure to look at multicast enabling (as suggested in other posts here) and latest firmware.