Actually, I'm willing to bet the problem is your Arris SB6183 cable modem, and not any of the settings on the AC5300. When you disconnected your old router and installed the new one, did you turn off your modem and then turn it back on again when you connected the new router? If you did, then Comcast may very well have reinitialized your modem and, if they changed the CM (cable modem) firmware in the interim from the last time you reset your modem, then you may have gotten a new configuration pushed to your SB6183 which has disabled IPv6.
Why is that important with the SB6183? And why would I suspect that Comcast has turned off IPv6 for your SB6183 (and likely not just you, but all SB6183's?) Because this has been an ongoing saga with that particular CM and most of the major cable ISPs. Late last year (2015), Arris discovered there were some major problems with that modem and many of the cable co's that had recommended the 6183 as a purchase option to their customers were forced to disable IPv6 on all of the SB6183's while they and Arris worked to find a solution.
Both TWC and Comcast had multiple issues with IPv6 and the SB6183. The issues involved several security flaws in the Arris firmware, including one that allowed a hacker to gain remote access through a backdoor in the modem (since there is no security password at all for the SB6183, and an even more serious flaw that apparently was so severe (according to what I've read) it potentially jeopardized the cable co's own internet network structure.
So both TWC and Comcast elected to turn off IPv6 for SB6183's late last year to wait for Arris to fix the issue. But the "turn off" only occurred if one reset one's modem (at least initially), so if you didn't reset it, the prior configuration, which allowed IPv6 would continue to work. So the fact that the ISP's were turning off IPv6 wasn't immediately noticeable to the end user, unless for some reason you happened to specifically check.
In the meantime, in December, January and February, Arris worked with the cable co's to develop two different new firmware solutions. Arris made these available to the ISP's, and in turn, Comcast was the first to push out the new "OSPREY" firmware to its customers, and Comcast had their IPv6 implementation for the SB6183's back up and running by March, 2015.
TWC however, never got their's working again and they continue to report (even as late as today) that they are still "testing" the new firmware and may push it out "soon", but they refuse to commit to any timetable. So on TWC, with an SB6183, there is no native IPv6 as of today. .
How am I aware of this? Because I'm using an SB6183 on TWC that I purchased back in early 2015 when TWC was recommending it as an alternative to their leased cable modems. I first heard about security the problem sometime in early February, when people were complaining at DSL Reports and at the TWC Community forums, that their IPv6 had been shut down sometime in December.
I then checked my modem, and in February found I was still getting IPv6. But then I reset my entire system, including resetting the modem because I had purchased a new UPS, which of course required that I shut down and restart everything, including the modem. When I rebooted the modem, IPv6 was gone, just like everyone else had reported. I've been folowing the issue for months, and TWC just refuses to issue any new firmware and will not say when, or if they will ever fix the issue for the SB6183. In fact, they've now taken it off their "approved" list of modems to purchase due to the IPv6 issues.
I continue to use the SB6183, and it's still a great cable modem for speed, i.e., it still "works" as a cable modem just fine, and I get like 320+ down and 25+ up on the TWC 300/20 plan, due to TWC's over-provisioning, but it's limited only to IPv4, and I have no IPv6 through TWC natively.
I had not recently heard anything about the SB6183 issues resurfacing on Comcast's system, but I would highly suspect that the issue experienced by the OP is due to just such a problem. The OP probably disconnected his SB6183 when he diconnected his old AC3100 and then rebooted it when he installed the new AC5300, and when that happened, Comcast probably pushed out a new configuration provisioning which likely shut down IPv6 on his SB6183.
In a perfect world, all that it should take is to get IPv6 working on any of the newer Asus routers is to set the IPv6 preferences to "Native" and "stateless" and to set the IPv6 address to be automatically obtained (or set an alternate DNS, for example Google's 8.8.8.8 and 8.8.4.4), and if it's available, then it should just plain work.
But I'm betting it's Comcast that has shut it down by hitting the SB6183 with a new configuration that has turned off IPv6 again. One surefire way to tell if it is Comcast is to check your SB6183's Event Log: Open a browser and type "192.168.100.1" and then go to the Event Log page and look for a line that says: "Notice Overriding MDD IP initialization parameters; IP provisioning mode = IPv6" (or something similar using the words "overriding MDD IP utilization" followed by "IPv6"). If you see that, then you know Comcast has turned off IPv6 again and is overriding the usual provisioning that the firmware allows.
I'm actually surprised to see something like this, but the fact that this involves a now non-working IPv6 with an Arris SB6183 cable modem that once worked and allowed IPv6 which now doesn't, is for me a giant red flag. I hope it's not the case, because that would indicate the SB6183's have bigger problems than anyone has previously made public, but I wouldn't be surprised. Arris' quality seems to be slipping. From what I've also read, they've had to push out at least four new firmware versions for the SB6190, which was supposed to be the replacement for the SB6183. Sheesh. Good luck to the OP, hope you get it working again.
