intermittent IPv6 on R6100 router

I turned off wifi and reran the test. The result was the same, aside from a slightly slower ping and download speed. I unplugged my Ethernet cable and also got the same result, except that with Ethernet I get about 30 megs download and with wifi only, I get about 24 megs download, which is why I use both. So, I doubt that using both Ethernet and wifi is the problem. I suppose it's possible that the Arris modem is at fault, but it seems more likely that it's the Netgear router, particularly since I have seen other people who can't get IPv6 to work on an R6100. I would have bought an integrated modem/router, but Comcast would not allow me to use one unless I rented it from them. There are only three settings on the IPv6 advanced setup screen. I'm using Auto Config with IPv6 Filtering Secured. I tried using DHCP, but that did not enable IPv6. The screen also shows the router's IPv6 address on the WAN, which may confirm that the modem is, indeed, supplying IPv6 to the router. The connection type is DHCP, which is why I tried switching to DHCP. Internet Connection Type is set to Auto Detect. On my Mac, IPv6 is set to configure automatically, and the IPv6 address of the router is shown. I'll look for another page to test IPv6 since the Comcast page may also be the issue.