Did you manage to get this to work?
I think that's because our operators doesn't support it. I am on O2 CZ.
Just wanna update you that with the WW_16.1220.1909.193_0 build chat features work now.
It's not a ZenFone 6 bug, it's an Messages.apk bug and it's on since I remember, for many many years.
I've had this issue since my first OnePlus One! :)
I've managed to get it work by puting a second sim card (borrow from family or friend). Then, reboot the phone and activate the RCS (Chat Features) on the first sim card.
After it's done, you can take off the second sim card.
Hope it helps!