Cannot access modem at 10.0.0.1 or 192.168.100.1

I had Comcast enable the Internet on my Xfinity modem because I thought my Motorola SB was flaking out. I cannot seem to get into the modem to check signal levels and whatnot and when I try to go to http://10.0.0.1 or 192.168.100.1 the page times out. I know the device is in bridge mode so not sure if that's the issue or not. I have a NetGear router plugged into the modem and my desktop plugged into the NetGear. If I plug my desktop directly into the modem it get my public IP address and I'm still not able to web to the thing. Any ideas?