If it’s any use, here are the WAN IPv6 options (auto, manual, or IPv6CP Extension) for the ZTE-ZXHN-F670, it’s set to auto right now:

And these are the available LAN settings:

I’m located in Vietnam; the ISP is Viettel (probably not useful information, but hey, why not).

Is it possible to send the hint from OPNsense itself? I mentioned in the original post that OPNsense is behind the ISP router (ZTE-ZXHN-F670) which I kind of don’t want to touch due to the fact that the terminology is quite different and that the ISP people come over to run tests on it whenever there are service issues. Either way, I’ve already tried to search through the settings and couldn’t find any options for setting the prefix hint on the WAN side. I’m open to digging though those settings again if you’re familiar with the router.

Thanks, I’ll look more into it.