You don't actually have to buy a second router upfront, so that's not a good way...

You don't actually have to buy a second router upfront, so that's not a good way to word it either.

I'm struggling to find a great way to put it. Maybe "a one-replacement backup plan gives you a 99% chance of success"? Close but not very elegant.

"Replacing your router (or firmware) fixes the problem except for 1% of all router buyers"?