I followed this guide: https://www.netgate.com/docs/sg-1000/faq.html#how-do-i-restore-the-firewall-os-firmware
And flashed the image to USB using this command: https://www.netgate.com/docs/reference/create-flash-media.html#linux-freebsd-mac-os-x
Then when I had the OTG cable plugged in, with the USB connected, it gave a re-flash option from the console on first boot.
Hope that helps.