Couple things to check:
1) Some routers allow the user to hide the SSID (not broadcast it) and you have to manually type it in on your wireless devices. Check your MOFI modem/router user manual.
2) Another option is to rename the SSID on the new modem/router to the original router and use the same wireless password as the original router...
3) Most wireless IP cameras use 2.4GHz, check your modem/router to ensure that the 2.4GHz (and 5.0GHz, if available) are turned on
4) If the cameras are able to connect up wired, connect one and access the cam web interface and check the wireless setup