The 192.168.100.x that the cable modem is giving out is actually due to the connection on the cable interface being down. The cable modem (specifically motorolla) will 'unbridge' when the cable connection is down and then give out an IP in the above subnet. You do not need to do anything usually, once the cable connection comes backup, it will register the pfsense wan interface 'mac address' to the dhcp server on the cable network and update the ip adress to the current wan ip given by the cable companies dhcp forwarder.
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