H Graham,
Thank you for visiting Microsoft Community.
From your description, I understand that the wireless printer is not working after replacing your router /modem.
I suggest you to follow the steps mentioned in the methods below to fix the issue?
Method 1: Refer the steps in the link:
Printer problems in Windows
http://windows.microsoft.com/en-US/windows/help/printer-problems-in-windows
Method 2: Uninstall and reinstall the printer
Add or remove a printer
http://windows.microsoft.com/en-US/windows-vista/Add-or-remove-a-printer
Note: The steps are applicable for Windows 7 too.
Install a printer
http://windows.microsoft.com/en-US/windows7/Install-a-printer
Please post back if you are still experiencing the issue. I will be glad to provide you with additional options available that you can utilize to get this issue resolved.