When you change ISP you need to reset the TC to factory and start over.. this is the router side not the disk.. the reset does nothing to data on the drive.
I recommend at the same time you do fix the naming the of the TC though.. use all SMB names, short, no spaces and pure alphanumeric. Then simply point Time Machine to the newly renamed TC drive which it will discover and use the old backup.
Universal Factory Reset.. any model TC or AE.
Unplug your TC/AE from power or turn off at the power point.
Hold in reset. and power the TC/AE back on.. all without releasing reset and keep holding in for about 10sec. (this is often difficult without a 2nd person or a 3rd arm).
Release it when the status light flashes rapidly. If it doesn’t flash rapidly you have missed it and try again.
Note..
Be Gentle! Feel the switch click on. It has a positive feel.. add no more pressure after that.
TC/AE will reboot after a couple of minutes with default factory settings and will wipe out previous configurations of the router.
No files are deleted on the hard disk in a TC.. No reset of the TC deletes files.. to do that you use erase from the airport utility.
