Factory reset ubifs

I've an HomeHub 5a running an old LEDE build - Reboot (SNAPSHOT, r3900-399d5cf) - that I want to reset. I originally used this snapshot link to install (around this time last year): https://downloads.lede-project.org/snapshots/targets/lantiq/xrx200/openwrt-lantiq-xrx200-BTHOMEHUBV5A-ubifs-sysupgrade.bin.

However, running firstboot per Soft Factory Reset returns MTD partition 'rootfs_data' not found. Presumably this is an different file system.

How can I reset this? Note, pressing down the physical reset button doesn't work.

There might be some LEDE/ BT HH5a specific info at https://openwrt.ebilan.co.uk/viewtopic.php?f=7&t=266.

If I recall correctly, I don't think you can 'factory reset' the ubifs builds. You'd have to reflash the router to completely reset it. And if you're going to do that, you'd be far better off upgrading to a much more recent build.