Are you using your address or landline telephone number when checking for a waiting list?
If you use your phone number and already have FTTC then it will always show available.
The waiting list only shows on an address check.
I know you don't have FTTC but I'd advise using the address check anyway.
Zen don't use the waiting list AFAIK.
Pretty much nobody does. It isn't a manual process and it's time consuming to manage.
After a set time period (i believe 3 months?) orders on the waiting list are automatically rejected and the whole process needs started again.
So ISP's just don't bother with it.
Any reported capacity issues aren't necessarily at the cabinet.
Although if there really is no cabinet I'm not surprised they can't find a free port on it
I would nudge Zen all the same and request an update.