What router do you have?
what firmware is on it?
What modem was installed?
All key details as you didn't give us anything to go off other than you changed modems.
There's a support site with access to manuals, firmware, install guides, etc.
Go to https://www.netgear.com/support/
Type in your model number and from there it'll take you to the above.
Its a great resource to start with.
