You can try opening 192.168.1.1 in your web browser, as for credentials you'll need to look on the router, sometimes the default username is 'admin' and some Internet sources say the default Calix password is blank. Otherwise you'll need to contact your ISP again and ask them.No I'm not sure where to login or what credentials to use.
Yes, in version 5 it is on the settings main page and it's a check box for "Register IP address to assist with client app connections". Also enter your BI license in the BI app and there is a button labeled "Get IPS" which will update with your current external IP address.Is that a simple setting in BI to tell the app when your external IP address updates?
I would think legacy 2.4 devices should still be able to connect. What trouble are you having? Do you only have one SSID being broadcast and can you connect to it with your doorbell and opener?
Have you tried to log into the new router? My guess is you can probably put the new router in bridge mode then connect and use your old router.