You might be able to do mac address cloning for fiber ports, if you have openwrt or similar it usually offers it.

A firewall may not offer full routing and NAT though for ipv4 devices, or wifi if you need it, or ipv6, or many other features. I’ve also used a cable modem and stuck the router in the dmz which essentially makes it a passthrough device also in the past

Basically, play with it and good luck!