Isn’t “MAC NAT” you are after? I’ve seen Mikrotik has this feature to perform NAT for bridge devices. EDIT: no, since your ISP might check at DHCP leases and realise that you are cheating. Go with regular router instead.
Also regular router would be sufficient IMO. Also don’t forget to set static TTL value so your “ISP” doesn’t see that you have a router between your devices.
Also create MAC address and save it. Always change it before connecting - you will have less trouble.