If you want to push routes to your client, the easiest way to do so would be adding a classless static route (DHCP option 121) as defined in RFC 3442. Every router has their way of setting these but usually they have one thing in common – you must do so manually. And yes, if you make a single mistake, your Internet connectivity will be lost.
Issue of easy entry has bothered me for long enough to actually do something about it. Below find classless static route option calculator. Just enter routes you want and you will get their hexadecimal representations.
[2019-12-13: Updated script to have default route first (workaround for Ubuntu 19.10 Server).]
[2020-12-26: Added OpenSense/Ubiquiti notation.]