When using an installer host in an isolated dualstck network, when the installer host is rebooted the IPv6 address is not the one that was assigned by openstack.The way to solve it depends on the RHEL version

RHEL8:

Add the /etc/NetworkManager/system-connections/required-rhel8-ipv6.conf file with content
[connection]
type=ethernet
[ipv6]
addr-gen-mode=eui64
method=auto

RHEL9:

/etc/NetworkManager/conf.d/required-rhel9-ipv6.conf
[connection]
ipv6.addr-gen-mode=0

After the change a reboot to the installer host is required. 

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