RBR20 blocking *something* with Amazon Firestick 4k

For the benefit of others, here is an update on the resolution to the problem.  In the end, this is not an Orbi problem, but one take-away relevant to Orbi is the use of the http://orbilogin.net/debug.htm page.

NETGEAR support instructed me to capture WAN/LAN traffic using the built-in http://orbilogin.net/debug.htm page and email it to them.  This is a great tool and what I was hoping for.  I don't know why it's not documented in the User Manual. 

They also requested that I put the router back to factory default and report back to them.  Resetting made no difference, but I took that opportunity to use a different wifi SSID and isolate all the other wired/wireless items on the network. I found the cause of the issue was related to having a TP-Link TL-R600VPN device connected in the wired network.  This device is being used just as a LAN switch (no WAN connected, DHCP disabled).  Here's where you can use that capture from the /debug page and really go to work!  I captured traffic both with and without that TP-Link switch in the network.

I saw in the failure logs that there is a lot of IPv6 communication from the FireStick, and seemingly a fallback to IPv4 at some point, but very slowly and it doesn't really recover. Including some link-local ICMPv6 neighbor solicitation/advertisements between the TP-Link and the FireStick, which I did not see in the success logs.

The success case logs showed all IPv4 communication from the FireStick, and of course no ICMPv6 exchanges.

Then I found a setting in TP-Link TL-R600 in the IPTV section that had IGMP proxy enabled (default maybe? I don't think I've ever even looked at that). I disabled that, and I believe that solves the problem.

I did not go back and capture new success traces to see if it is all IPv4 or if it is IPv6 working better.  I understand that IGMP can affect exchange of IPv6 info info but my understanding of what exactly is happening is a bit shaky. I also can't explain why the Firestick had issues with some apps, while other apps were fine and all other devices on my network were fine - including an AppleTV running exactly the same apps that fail on the Firestick.  I assume it comes down how the devices/apps are using IPv6, but since it is working now, I'm not looking any deeper at this point.

Thanks to all for the input and help.