comments:
It you want flexibility to move the modem around use this https://a.co/d/04uOwBvB
If its just the thin white cable with the green connectors, you can buy a replacement on Amazon. I used one of these to move my ONT from my garage to the wiring cabinet in my closet. You can get them in all sorts of lengths https://www.amazon.com/dp/B09XX4366J
By "fiber box" do you just mean the ONT? Assuming it's a standard fiber connection (probably a green SC/APC connector), you could buy a coupler and extend the fiber to the network rack's location, and install the ONT there with all the other gear. https://www.amazon.com/dp/B09XX4366J/ or similar. I saw another comment here about buying SC to LC adapters, using one line of a duplex LC cable to extend the fiber to the new location, and another adapter to get back to SC for the ONT. The other half of the duplex cable was then used with bidirectional SFPs to run a LAN connection back to the original location to install an AP or other gear. I thought it was a clever use of duplex cable to relocate the incoming fiber and still provide access at the original location.