Sadly, Zen has dropped support for RFC 4638 as of a week or so ago.
I know this for sure on ADSL, not too sure about Fibre.
I had an email from Zen confirming there will be an "upgrade" and as such my internet may wobble for a bit about a couple of weeks ago, since then my router will not use RFC 4638. Setting it by force causes packet loss.
Called Zen's technical team earlier and confirmed they have dropped RFC 4638.
No idea why they would downgrade their service in this way. I asked if they would ever switch it back on and they said no.
I am posting this to stop anyone spending hours messing with OpenWRT wondering why they can't get this functionality working. Its them, not you