I have a business internet connection with Spectrum where they via a single cable get me both DHCP access to some IP of theirs as well as access to a /29, and their device deals with both. The thing is I don’t want to use their device. I know I could use their device as a pass-through by just turning the firewall off and then sitting a pfsense box inside of it. What options are there for a firewall where I set up some LAN interfaces on the /29 and others on the plain old DHCP network?
You generally do either one but not both. If they are assigning multiple IP’s via DHCP then each IP would need a MAC address. The better option would really be to put their equipment in bridge mode and set the IP addresses up statically. This is how we configure our clients with static addresses.