Login to your CentOS Server and check if IPv6 Router Advertisement is enabled via
sysctl -a | grep "net.ipv6.conf.eth0.accept_ra"
If not, enable it using
echo "net.ipv6.conf.eth0.accept_ra=1" > /etc/sysctl.d/enable-ipv6-ra.conf && sysctl -p /etc/sysctl.d/enable-ipv6-ra.conf
While trying to get my EC2 Instance to work with IPv6 I tried everything from recreating instances, adjusting routing tables and subnets. Nothing worked.
The EC2 Console each instance had assigned 3 IPv6 addresses, that I assigned beforehand but I could neither reach the server nor could connect to other IPv6 services.
Pinging the domain via ping6 results in connect: Network is unreachable
With this error I came across a bug in Centos 7 which explained that Router Advertisement is not enabled by default for CentOS.