A quick search suggest there's no WRT firmware for the device. That's a pity, OnHub has 1G ram and I can imaging a lot of creative usages if WRT firmware is available.
If anyone is reading this and has an OnHub they want to donate, just hit up the OpenWRT dev mailing list and offer it to existing devs. The OnHub is an ipq806x device and should be easy to support.
It's likely expecting to boot from a custom vendor kernel, even after being placed in "developer mode" - which is probably why the "rooting" procedure is so involved. Look, I get that this has a nice amount of RAM and all, but a lot of this random IoT junk is just not worth supporting.