As mentioned elsewhere in the thread, Fiber works fine in a power cut as long as you can power your ONT and router. The local cabinets are passive and the rest of the network elements have backup power meaning that, for a while at least, you can continue to use during a wider power outage.
If you can keep your ONT and router going for, say, 4 hours during a major power cut, do you think that fibre broadband would also be able to keep going for that long?
yes, absolutely, those with just batteries will likely get a generator shipped in for a long power cut, and others have batteries and generators already there so just need topped up with gas. so they will last indefinitely.
you will likely run out of power before they do.