Fiber optics internet connection

I suggest that you work with your service provider to understand what modems or gateway devices that they support. With Apple getting out of the networking hardware business, you need to consider using other manufacturers' equipment going forward.

However, if your desire is to continue using Apple networking hardware, just have your ISP to provide you with a simple modem that you can connect your AirPort base station to via Ethernet.

Apple have not made modems.. dial-up analogue, for ages.. they make nothing suitable to connect to fibre directly.. it must be ethernet connection from a suitable ONT that would be supplied by ISP.

But Apple routers are not really suitable even with ethernet connection in a lot of situations.. it will depend on exactly how your particular setup is configured.. if you tell us where in the world and the supplier it might help us determine special conditions.

Apple routers can be bridged and used in a network with the ISP supplied modem router although as Tesserax noted.. Apple are now out of the router business and if you have a new network configuration it is also time to look at alternative wireless routers for the job. Ask if you want some suggestions.

Your service provider may tell you that the router they give you supports "pass through mode," a setting accessible from the device's internal web configuration page. Using pass through mode allows your Apple equipment to function as the router eliminating a problem known as double-NAT.