How to route traffic over specific interface on linux (debian)?

I would like to route traffic over usb0 (a cellular modem) when it is available instead of eth0. The ip address for usb0 may change over time. How do I create a route in this scenario since the ip address may change? I tried: ip route add default dev usb0 but when I do this, I am able to ping from usb0 but no longer through eth0. Any thoughts why? Destination Gateway Genmask Flags Metric Ref Use Iface default 172.24.1.1 0.0.0.0 UG 10 0 0 eth0 172.19.200.136 * 255.255.255.252 U 0 0 0 usb0 172.24.1.0 * 255.255.255.0 U 0 0 0 eth0