This is a relatively confusing scenario, so I will try to lay it out as clearly as possible.

My family is currently staying with my uncle at his bach. He has a phoneline there, administered by Telecom, and connected to broadband, on a very small account. We were going to be staying here for an extended period of time, and would be using quite a lot of data, and so we thought we would try using our home internet connection, since it is not in use at home while we are away. This is administered by Telstraclear. We weren't sure it would work, but we figured we'd try and see.

A new modem was purchased, (TP-Link WD8960N), so there is no issue with hangover of old settings. This was configured with our TelstraClear settings, and to our great delight, it seemed to connect and work no problem.

This was 6 months ago.

This Christmas, the same scenario occurred, so we got that modem out (the settings in it had not been touched, it had just been off for a few months), plugged it in, and we were away.

Today, my uncle received an email from Telecom, informing him that he had used 80% of his data cap - a huge surprise to us, as we didn't believe his modem (with his username and password) was in use at all.

I logged into the modem, but unfortunately, I assume as a security measure, the modem doesn't allow you to see what username/password it's using to connect to the ISP. I reentered the TC username and password, and rebooted the router. The external IP address now certainly seems to be from TC (203.97.210.xxx). Unfortunately, stupidly, I forgot to check what it was before I reset the settings.

It has been the best part of 10 years since I have worked in tech, so things may have changed. Does it actually now not matter what username and password are in the modem, and is it somehow just charged to whatever account is in charge of the phone line? Could the modem somehow have received "updated" ISP details from the cabinet or ISP? Is it likely to be working ok now, and I just have to check it each time the modem reboots? Is it possible to be connected through a TelstraClear gateway, with TelstraClear usernames and passwords, and for the data to be being applied to a Telecom account? It appears, over the past few days, that there has been no usage applied to the TC account at all.

I realise this is confusing, I am happy to try to clarify anything further that I can.