Hi all -

An elderly family friend was recently setup on OneNZ wireless broadband with the Deco X58-4G after copper was removed in their area.
Their landline number was migrated at the same time.
His desktop PC is hardwired into the new modem, the phone port goes to a nearby landline phone and his mobile connects to it over WiFi. This all works fine except we can't add new devices.

From what I've been told, it sounds like OneNZ shipped him the new modem, and later sent a technician out to complete the install.
The tech took the packaging and rubbish away which was nice, but it sounds like the default WiFi password likely went with it.
Unfortunately we can't connect his smart TV as his previous wifi password doesn't work, there's no password on the base of the modem and as all the packaging has gone we can't check the box either. 

I connected to the web portal from the wired desktop, and it asks for a Deco owner password - don't have this.
On this web-portal and on a wifi device, we've tried the password as the SSID, IMEI, Serial#, previous password, all without success.

We've reached out to OneNZ support who said to download the Deco app and login, we've downloaded the app but he has no account so can't login.
Creating an account still won't let him link the modem either. It seems to be setup already and locked to whoever did it?
OneNZ support also suggested a factory reset. This appeared to complete but we still get the same prompts via the app and web portal, locked to whoever set it up.

The chat support from OneNZ kept saying download the app and login, and don't seem to understand why that isn't working. 
Online guides from TPLink also suggest the same thing, appears to all be done via their app with no way around it when that doesn't work. 
Does anyone else have experience with this modem and know how to get around this?