Thanks, I got sorted.
Set the internal NIC to be a DHCP client of my current router so that the Pi had internet access
Then connected to it's IP address and installed the package in the Software section.
Previously I had been offline and manually trying to upload the packages, and getting errors.
you can use the https://firmware-selector.openwrt.org/ to create an image with the extra packages you need (like the eth driver), but it'll also reset the size of your aprtitions, if you resize them post install.