The way you describe it, especially with it working when you disable IPv6, it sounds like being related to the very old peering disputes between providers (these are not new, and I have not followed the development in recent years, but I can imagine they are still a thing).
I have found this workaround, which is of course also not a complete fix. Basically, you figure out which IPv6 prefixes are not being routed to the end destination (in the example from the link, these were all Cogent prefixes), and then disable routing for these prefixes, like this (copied from example above):
#!/bin/sh
for prefix in 2001:0550::/32 2001:067c:12e8::/48 2001:0978::/32 2607:9700::/32 2607:f298:000a::/48 2607:f5d8::/32 2610:00f8:2f00::/48 2610:00f8:2fed::/48 2620:009a:8000::/48 2620:00fb::/48 2620:00fb::/56
do
ip -6 route add unreachable $prefix 2>/dev/null
done
exit 0
The correct fix (after you verify this is indeed the problem!) would be of course to complain to your provider and hope that eventually these disputes will be resolved for good. You can pinpoint the exact location of the problem by running:
mtr -n -6 <ipv6_address>
from your machine and from a machine that can reach the address (You can omit the -n of course, but I prefer to have the numeric addresses, and when figuring out where the problem is, find out the name by a whois <ipv6> query)
Now the next question is, why does Chromebook and Windows work on the same router. I can think of two possibilities here:
- They have already IPv6 disabled for some reason.
- They use a different DNS resolver (default resolver from your Fiber provider, and in your Ubuntu box you have set up some public resolver like 8.8.8.8), which may be configured to filter out IPv6 routes that it knows you can not reach. (A little far fetched, but possible).
The reason why it works if you use the Google mesh router should be similar, that router is probably not set up to give you a public IPv6 address, so basically it causes your network to have only a link local IPv6 address and not able to reach any public one, which makes all connections fall back to IPv4.
