It is true, that ISPs tend to go their own way on supplying DOCSIS based internet.
The basic idea of IP assignment of a DOCSIS modem ( CPE ) is
- while the connection to the CMTS isn’t up, the modem works as a DHCP server in the 192.168.100.0/24 network ( lease time short, 30s for example )
- when the connection is established, the DHCP server stops. DHCP requests are forwarded to the ISP DHCP server.
- renewing the private IP with short lease time, results in the detection of the internet connection by a RENEW/NAK/DISCOVER/OFFER sequence resulting in a public IP.
dhcpcd handles this procedure right.
Problem is that some DHCP servers of ISPs do not implement the simple algorithm explained above.
ISPs further tend to not providing a simple modem ( or a router with bridge mode ), but router with its behaviour of its own ( defined by ISP ). A router should handle the process much easier ( supply a private address, which doesn’t change independent of the online status of the WAN side ).
Obviously there are problems on the ISP side, which should be solved on the side. There is no save procedure in IPFire to circumvent these problems. My opinion.