If you had a static IP address setting for your ISY, that maybe the problem. It needs to be changed to DHCP reservation from your router.
Even though your old router may not be fully functional, if you still have it, try connecting it to your ISY to see if it lets you connect. You don't need Internet access, just local Lan.
If you can get in, go to configuration tab and enable DHCP.
Also, here is some information from the wiki, including an option to factory reset your ISY if you can not get in any other way - however, you will need a good ISY backup.
https://wiki.universal-devices.com/Main_Page#Changed_ISP/Router_-_Can_no_longer_access_ISY