I still think it is a network issue more than anything else.
Multicast can be finicky and there are plenty of things that can make it not work as expected.
If you are up for a little experiment, can you use your old router to have a dedicated network only for the AVR ?
You can continue to maintain Internet connectivity on this dedicated network too
Basically have something like WAN - Modem -> New router (on an IP range of say 192.168.1.x/24) ----> WAN port of old router -> LAN for only AVR (On an IP range of say 192.168.2.x/24)
You can then connect your phone to the old router wireless - test for a couple of days to see if it runs stable
I have a strong feeling something else on your network is causing a multicast flood - Airplay in particular is very prone to drop offs in such cases
Isolating the AVR on a separate subnet will eliminate that intereference
If your setup runs stable with the suggested experiment, you will have the option of isolating the issue once and for all.
PS: Also check if your new router has an option to enable IGMP Proxy or IGMP snooping - enabling this will help with proper multicast routing
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@Love4sound - LAN traffic issues can be difficult to diagnose and don't manifest in a very straightforward manner. Believe it or not, I had massive delay/dropout issues with smart bulbs to the extent that I replaced quite a few. The problem was eventually (and surprisingly ) traced back to DTS Playfi receivers)