Eric,
your explanation is only for a DOCSIS router ( with own NAT ). A real DOCSIS modem just passes through the communication with the DHCP server at the ISP ( modem works as bridge). The modem has its own local IP at 192.168.100.1 ( the net 192.168.100.0/24 is used by the modem’s DHCP server in case of failures on the cable side ).
My experience is, that the actual DHCP client ( dhcpcd ) in IPFire has some problems with DHCP servers not responding correctly to his requests and thus doesn’t notice a change in the assigned public IP address.
EDIT/UPDATE: This change of IP happened again this night. No internet connection. A
dhcpcd -k red0 ( release interface red0 )
dhcpcd -n red0 ( rebind interface red0 )
solved the issue.