You don't have to change the AX6 IP address. Leave it as is. Just set the "other" router to 169.254.31.1 (I actually didn't use another router but rather hostapd on my laptop).
The two routers are connected to each other through Wi-Fi. This is happening when you browse to the extendwifi_connect URL.
The PC is what causing the AX6 to connect to the "other" router. Keep it connected through LAN, and by browsing to the extendwifi_connect URL you'll trigger the AX6 to connect to the other router through Wi-Fi.
And yes, "on the AX6" means that you just have to browse on your PC to the AX6's IP and URL, i.e. the URL that you provided.
I received my AX6 today, of course all Chinese language. The webgui starts with some sort of a wizard, but it seems there is no way to skip it and get to the main menu? Hilariously annoying. Intentionally I dont want to set up WAN yet, but it seems this crappy wizard cannot be skipped....
Actually don't remember exactly, but as far as I remember there wasn't something long. Just the name of the network and password and no long wizard (maybe some notification). It took me literally few minutes to tune it.
The only problem I had was wrong MAC of previous router, but that was my fault.
Managed to set it up, upgraded to 1.0.18, FYI still no English language in it. The whole webgui is way too primitive, it is clear that this device badly needs OWRT.
If someone wants to open it: the screws are behind the sticker at the bottom, after removing those, the top plate is still held in place by little plastic clamps, but it can be gently opened with a guitar pick or similar tool.
Going to be honest: I have not seen such a bad editor as OWRT wiki has... Trying to format the MTD table to look properly but apparently that surpasses my abilities...
Tried to share as much useful data as it is possible. If anyone needs more data that can help the development, please let me know and I can upload that as well. Wonder a dump on the MTDs might be helpful too.