Your last two points contradict each other. If it's not urgent, they can be gone for a few hours or days. If it's a consistent problem, that's a different discussion.
In an office, someone will fix it if the internet is down. That's why you have an office: to outsource infra to people so you can focus on your own job: writing, coding, whatever it is.
If you're remote, your internet being down is _your_ problem and you're expected to figure it out. You have more responsibility—and that was an unexpected part of working remotely for some people.
If the company REALLY wants that to be my responsibility, they can pay for me to run a 2nd ISP line, or any other myriad potential solutions.
Even in-office, most places don't run backup ISP connections, the internet goes it, it goes out.