You can/should control your routes by the metric. You can/should have your secondary default route added with a higher metric value than your default route. If the default route is inaccessible, it will use the next route for 0.0.0.0/0. When the gateway/cable modem becomes accessible once more, the default route is chosen since it's metric is lower.
I'm sure that openrc can set this up when the network is brought up, I've just never looked for the way to do it. Now that I think about it, I know I read about this very topic in when I used Gentoo and it's version of openrc. Check the do