So you have a fiber ISP.
Are you sure that the fiber is directly connected to the ISP's modem/router, or is the fiber (safely) terminated at a Fiber Termination Unit (FTU) to which then the ISP's router connects with a plain ethernet CAT cable.
If the latter is true (with a FTU) then you probable could very well connect your own router to the FTU. Likely you still need to know any connection parameters like VLAN setting and, possibly, PPPOE config. In the whole of the EU the IPS is required by law to supply these connection parameters on request (although some ISP are truly a PITA). That would give your own OpenWrt router direct WAN access.
When you're not that lucky, then lets assume that NAT on the IPS's modem/router is something you have to live with. Then you could still choose from different options:
- Run your own OpenWrt equipment as an access point (a LAN bridge, no routing) behind the ISP modem. Access to services running on your own stuff from the outside Internet requires setting forwards (or set "DMZ host" in the modem/router of the IPS. All services like routing, NAT, DHCP, firewalling is handled the the router of the ISP.
- A little bit more evolved, your own router as an downstream router on your LAN. This allows creating subnets, VLANs, etc behind yours. This setup requires that downstream routes for any subnets behind your own non-masquerading router can be set on the modem/router of the ISP. Nice thing about this setup is that you will likely enjoy the full speed of the hardware NAT engine that it deploys. Breaker is when the NAT facility is "hard wired" to the subnet of the IPS modem.
BTW, when your ISP deploys GCNAT (part of DS-Lite), then:
- the second option will be impossible because you cannot set routes upstream.
- you will need some tunnel via some proxy to be reachable from the outside over IPv4.
Hope this helps.