This is a bit of a niche one, not least because network-scripts is supposed to be on its way out as a configuration method, despite cPanel still not supporting NetworkManager properly. No problem people, it's only been around for ten years.
This may also be specific to OVH's network, but I haven't found a resource that covers it, so here are my notes.
Dedicated servers at OVH are allocated a /64 IPV6 range. The gateway is the last address in the range, which is slightly more eye crossing than with IPV4, but ends in FF:FF:FF:FF. However, in EL8 (and in Debian-derived distros to my knowledge), ifup doesn't add a default gateway if it can't ping the address, and it can't ping the address because it needs a default gateway. This also applies to IPV4 in OVH's network, which is a pain for automatic provisioning.
The network-scripts system is remarkably flexible, and wrangling it is a dying art, but the key here is the ifup-post script. This runs after ifcfg-<device> and goes through a sequence of the other scripts in /etc/sysconfig/network-scripts- you can see which ones if you look at the code.
One of these is route6-<device> and this is where you add your default route so it's added after the initial network config. In OVH images it is disabled, and perhaps confusingly, includes a list of static routes generated by cloud-init. Rename that one, and using your editor of choice (vim, you animals), open a new file vi route6-eno1 (for example) and enter your gateway and default route like this:

2001:xxxx:xxxx:xxff:00ff:00ff:00ff:00ff dev eno1
default via 2001:xxxx:xxxx:xxff:00ff:00ff:00ff:00ff dev eno1

Enable the script by making it executable chmod +x route6-eno1.
and restart the network with systemctl restart network.
and you should now be able to ping the IPV6 host of your choice (let's face it, your local gateway).

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