Something weird happened with my ISP changeover today, and I wonder if I describe the symptoms, if anyone can suggest what might be the problem? The three players involved, Flip, Chorus, and unlimitedinternet.co.nz (plus me makes four) don't seem to know the cause and are all pointing fingers in other directions.

So here's the scenario:
I live at an address which has 2 houses, separate households, on one single postal address.
I was connected to FLIP, but because I want a static public IP address, I decided to switch to unlimitedinternet.co.nz (hereafter UINZ).
During the process, UINZ, contacted me and explained that because there are two lines coming into this address, they need the ASID for my line. I emailed FLIP, obtained the ASID number, and emailed it to UINZ. So far so good.

Today the change happened while I was at work.
I got home to find that my internet was still up and connected, and hadn't been disconnected, as I was on a dynamic IP with FLIP, plus they use CGNAT, so it's usually a different IP address every time I check. This time, my IP address was unchanged since this morning, and I was still connected to FLIP.

No drama. I logged into my router, and changed it to the correct settings for my new ISP. PPPoA, 0/100, VCMUX, new username and pword.
Saved settings, rebooted router.

The router found sync, and connected to the internet. But it still had a dynamic FLIP public IP address, issued by FLIP. Weird.
Rebooted router. Again, a new dynamic FLIP ip address. (I am talking about the public IP, the one returned if you go to what's my IP.com.)

Called UINZ, who advised that I do a complete factory reset of my modem, and redo the settings from scratch.
I did so, making sure I put the right settings in. Rebooted. 
Again, connected to internet with FLIP issued public IP, even though I was using my UINZ username and pword.

Called FLIP, who said it might help if they completely cancelled my account, which they did.
I rebooted the router again. 
This time, it found sync, but was unable to connect to the internet, coming up with an error of "authentication with ISP failed".

Spoke to UINZ again, who said it might be one of the settings in my modem, or a Chorus problem, or my neighbour's problem, etc.

-Neighbour's (same address/section) internet is fine, hasn't been touched.
-My modem is new. TP-Link TD-8840T. Worked fine before today.
-The settings in the modem have all been checked and double checked. There isn't that much to change.
-Why was I still connecting with a FLIP IP adr. after the change had been done?
-FLIP turning off my account in software stopped me connecting to the internet.

These things make it seem likely to me that the problem is at UINZ's end, as I've pretty much ruled out my modem, FLIP, and Chorus.
Any other ideas? Cheers!