1-5: You need to advertise dns for your deviced:
IPv6 → ND
6: Yes, that is normal.
DHCPv6 is ROS is currently incapable of handing addresses - only prefixes.
All the addresses on another RB have to be configured manually, got by SLAAC, or picked from prefix pool.
So you can:
assign the address manually on the interface facing first RB with advertise=no
DHCPv6 server on the first RB to hand out the prefix.
DHCPv6-client on second RB to get the prefix
address from pool on the LAN interface of the second RB with advertise=yes
Clients on the LAN of second RB will get their addresses from the prefix handed by first RB
I’m already advertising DNS and both my devices (non RB) got it, as well as an IPv6 like prefix:subnet:MAC-Address, but I can’t open only IPv6 sites.
I’m also using IPv4 for LAN, maybe that can cause problems?
I read about DHCPv6 but I didn’t use it during my tests.
The problem with “native” 6to4 is that you can’t choose which relay to use, and if the one you are connected to is not working properly, there is nothing you can do about it.
You don’t even have a way to figure out whether the problem is on your side or not.
I use HE tunnel and don’t have any issues with it.
You can try it, at least to rule out the possible misconfigurations on your side.