In the US at least, I haven’t heard of a provider not allowing you to bring your own hardware. For example, on Comcast, you can bring your own DOCSIS 3.1 modem that isn’t as full of software and doesn’t allow neighbors to use an Xfinity public network. Comcast even has a page for compatible devices. Save on stupid rental fees.
Surely, one of you fine folks has a better solution to attach a spare fan to an ISP gateway without damaging it? ( sh.itjust.works )