@Jamval @IrvSp 

I had the unfortunate experience of my AT&T Fiber service going down last Thursday afternoon.  Friday morning a tech came and fixed my damaged indoor cabling (my fault for tinkering with fiber).  Working on the problem most of the day every day forward, unless doing other chores, eating, or sleeping, I finally resolved the issue this morning with the help of a cheap router that made troubleshooting easier.

At least for AT&T and the Tenda RE6L Pro, the two settings are DHCPv6 and DHCPv6.  I believe this translates to get IP address assigned by the ISP.  (I can't see the menu as the Deco is disconnected).  So SLAAC+Stateless seems incorrect and if you switch to DHCPv6 maybe that will resolve the issue....

  @Jamval 

This IS very Interesting!!!!

As you might recall, I'm on Spectrum (Charter) as well. Before, on TEST-IPv6 I also got 0/10 prior to try today. Got what you did. 9/10 and " Your DNS server (possibly run by your ISP) appears to have no access to the IPv6 Internet, or is not configured to use it. This may in the future restrict your ability to reach IPv6-only sites."

So IF that were truly the case, NO IPv6 test would have succeeded?

I suspect this is due to how a 'stacked' DNS would work? It tried IPv6 address first, fails, the IPv4 address is used. Seems that the test site had 'seen' that be used on the site, so that is why that line was produced? I'd ignore that.

On that test page, there is another link to test sites, and I picked this one, test and got 1 out of 10 for the results? Very similar format of the testing.

I think it is more possible that the site was changed to handle Stacked DNS's and not all were?

  @Jamval SLAAC+Stateless DHCP seems to work.  test-ipv6.com shows my ipv6 has been detected, with a 9/10 score. The test results show that my DNS Server/ ISP (Spectrum) appears to not be able to connect to IPv6 web pages, but that is contradicted on the tab that has a lengthy list of ipv6 URLs, I can connect to all of them.

The downside of all this was that I thought one of the benefits of IPv6 was eliminating the need for NAT in IPv4 networking.  Eliminating the need for NAT is supposed to speed things up a little, like reducing latency on streaming devices like smart TV's and Fire Sticks.  After enabling IPv6 I noticed I was getting a lot of video buffering on my streaming device that I didn't have with only IPv4.  I ran a speed test and wasn't getting anywhere near what I got with IPv4.  I ran speed tests from 2 PCs with wifi and with ethernet and maxed out at 80 Mbps.  My Spectrum service is 1 Gbps.  I disabled IPv6 in the Deco app and rebooted modem/router and now my speeds are back up to where they should be.

I googled AI with this situation and got a few suggestions.

1.  "Try DHCP6 instead of SLAAC+Stateless".  I tried this, rebooted, test-ipv6.com failed, no IPv6 address detected.  Speeds are where they should be.

2.  "Even though SLAAC+Stateless enabled IPv6 address to be detected, Spectrum's implementation of IPv6 in some areas is poor.  Change DNS Address to Manual and enter the google and cloudflare ipv6 addresses"  I didn't try this, not sure of security consequences.

3.  "Disable QoS in the Deco app".  Mine was not enabled.

I have now disabled IPv6.  The conclusion I've drawn from trying several options is a vote in favor of the old addage:  "If it ain't broke, don't fix it".

The only benefit is that I got in some needed stair-stepping physical exercise, my modem is in the basement and Deco router up stairs.  Several rebooting trips up and down.

Side Note:  When I upgraded to Spectrum 1 Gbps service last year they gave me a free router. I think my Deco's are compatible, but before I got that far I tried the Spectrum router alone, which to the best of my knowledge defaults to dual stacked IPv4/IPv6.  Everything worked except for some of my streaming device apps failed to connect.  I googled AI this situation and the response was "Many Spectrum users report this issue. Spectrum's default IPv6 implementation blocks some device apps.  Go into the Spectrum router web interface and disable IPv6." 

  @Jamval

@IrvSp 

I had my internet service go down Thursday afternoon.  After the tech left I realized I was not getting IP Passthrough service to my ASUS router.  Eventually tried the BE95 and failed with that too.  So I finally was able to resolve with a Tenda RE6L Pro.  While I had the RE6L Pro connected I got IPv6 to work.  Now I have the Deco BE95 connected and it also works, 10/10 on that ipv6 site, BUT I had to change one setting on the Tenda to DHCPv6.  No surprise I do not think I can upload the .HEIC image from an iPhone, I'll just type the settings out:

IPv6:  toggled ON

Internet Connection Type:  Dynamic IP

Get IPv6 Address:  Stateless

Prefix Delegation:  toggled ON

DNS Address:  Auto

*Assigned Type:  DHCPv6

*For some reason I thought this setting should be SLAAC+Stateless but that appears to be incorrect.

As reference I am in a suburb of Los Angeles city of Arcadia and this is for AT&T fiber, which doesn't seem to work in all areas AT&T services.