Internet connection on a linksys mr8300 router, behind a netgear nighthawk cable modem

I have a linksys mr8300 router that connects to a netgear nighthawk cable modem just fine with the default OEM firmware installed, and allows client devices connecting to the linksys to connect to the internet without issue. Leaving the router/modem configuration in place (linksys is connected to the netgear modem via an ethernet cable to the "internet" port on the linksys) I've flashed the linksys mr8300 with openwrt factory and sysupgrades, then set up/enabled the wireless radios on the linksys. I can connect to the linksys via wifi just fine, but am unable to access the internet via the wifi connected devices, and when sshing into the linksys, it's clear the linksys router is also not connected to the internet.

I'm assuming this is probably a very newb issue to run into, and though I've read through the documentation most tips are about setting up PPOE on the router which doesn't seem right. Any thoughts, tips, or feedback much appreciated!

As a quick thing to try change the routers IP address from to
NetWork > Interfaces > and edit LAN

Save then Save and Apply and reboot

Thanks - Ok, just updated the router's IP to and rebooted. No luck. Interestingly if I ssh into the router and try to ping the cable modem (assuming or, both return "ping: sendto: Network unreachable".