On fiber? There's usually no modem to buy

Cable modems are a cable-internet thing. Fiber services terminate in an ONT (optical network terminal) that the ISP supplies and manages — there is generally no customer-owned ONT program. If your ISP is fiber, the buy-your-own-modem savings play mostly doesn't exist.

What you often CAN still own on fiber

The router. Several fiber ISPs let you put your own router behind their ONT — that gets you better Wi-Fi and your own network control, though it usually doesn't remove an equipment fee the way an owned cable modem does. Check your ISP's page here: where the ISP itself says routers are permitted, we quote it.

Why we mark fiber ISPs "not allowed" rather than hiding them

Because the honest answer to "which modem should I buy for this ISP" is none, and a site that answers a buying question owes you the no. Buying a DOCSIS modem for a fiber service is a guaranteed return — we'd rather you didn't.