That is not the default value, something must have changed this at some point.
If the ISP DNS is down, then you most likely have a problem anyway, unless you were overriding the ISP DNS. Full ISP DNS outage is very rare however, assuming they properly implemented redundancy, and didn't have both DNS IPs point at the same physical DNS server... I would expect them to resolve these types of issues very quickly anyway, so your router will only report the connection being down for that period of time.
If you had enabled SSH then try accessing over SSH.
It's a new feature Asus recently added to encourage people to use https instead of http for security reasons. Quite bluntly, blame it on "security experts" who open CVE issues whenever a router defaults to http, leading to manufacturers implementing these kind of "security enhancements". I haven't studied Asus' implementation, but it's possible that if you set your router to HTTP-only instead of HTTP+HTTPS, it might go away. Otherwise, start using https when accessing your router.
Cake is a QoS mode of its own, it was never supported by Adaptive QoS. If you are thinking of fq_codel, that was removed last year because it was no longer possible to hack fq_codel on top of Adaptive QoS due to changes Trend Micro did on their end.