I have replaced my isp router with the mt6000 believing that it had a bridge mode but it looks like there is no way of doing it natively in the UI. Is there any way of putting it in bridge mode so it passes all the ipv4 and ipv6 to my second/main router to handle?
Use the Extender network mode. See Network Mode - GL.iNet Router Docs 4
Bridge mode and still acting like the router itself isn't supported.
I use the mt6000 connected to the ISP's ONT to obtain a public IP address in DHCP on the WAN interface as DHCP requests must be accompanied by an "option 60 vendor class identifier" imitating that of the ISP's router. How can I pass ipv4 and ipv6 trafic without any filtering or NAT functions to my second router?
I would say you can't