Remove the internal sd card make sure you completely wipe it inc all the partitions.
Flash using Balena Etcher.
Extend the ext4 partition with diskgenius or any other partition tool and your be good to go.
Download the update opk file.
Put them in a folder called apps on your external SD card.
Launch your device, go over to apps and open the updater. Follow the instructions and you're done.
RG-350m Alternative Firmware (RG350m-ROGUE)
This firmware aims to improve the user experience and includes support for a USB WiFi dongle, has a app store and includes an updater. (Requires WiFi)
It gets reasonably frequent updates. (About once every few months)