My ISP (Slingshot) had provided a Modem.  I presumed that the Modem was linked to this ISP and that I would need to keep using the same Modem.  I have a Chorus fibre ONT.  I had a problem with the Slingshot Modem and couldn't access the Internet. However, I had a spare Spark Modem and did a factory reset on this.   After the reset, I was still able to have access to the Internet via my original ISP (Slingshot).

How is this ?  Are the ISPs details stored in the ONT ?  I have several other Modems (ex Trademe) that were originally with various other ISP's.  If I try resetting these, am I likely to end up with the same result.