Dealing with the same thing with AT&T fiber. There was word of a hack involving putting your router behind a switch with the AT&T router after cloning the MAC, then booting them both up and letting your router pick up the DHCP responses along with the AT&T router. Once the AT&T router had done its proprietary handshake, you could disconnect the AT&T router.

Unfortunately I had no luck with that - my loose theory is that my EdgeRouter was doing a ping check to see if the IP was already taken before accepting the DHCP lease...

I was able to get "IP passthrough" mode working with the AT&T router though. The key hiccup was that the AT&T router had to be on a different subnet than my router's LAN subnet.