USB tethering connectivity

struggling to get a backup wan going with usb tethering. i'm running a snapshot build on a nanopi r4s, android phone (android 9, so v old) attached via USB. openwrt can see the phone, creates a usb0 interface, i set it as DHCP client and i can ping the phone itself (i.e.

default via dev usb0 proto static src metric 20

but attempts to ping the outside world from the router (e.g. ping -I usb0 fail.

When I plug the exact same phone, exact same setup into another device (Belkin RT3200 running, everything works. The R4S has the main WAN connection on it too, whereas the RT3200 is being used as a dumb AP so has no other configured WAN connection.

Am I missing something?