I have been using all sorts of Zyxel devices like Access Points and Switches but I've never hit such a painful experience with my NWA55AXE that I recently bought second hand.
First things first, the device does NOT have a factory reset button at all. So I got my device that had been administered by what they call the Zyxel Nebula administration interface. It is some obscure cloud shit that nobody should ever use tbh.
So how do I flash OpenWrt? Happily the device fetches an IP via DHCP and there is a WebGUI. Nice? No, not at all. Even though I could login with the default credentials (admin/1234) there was nothing I could change except the IP settings. The WebGUI was completely stripped down to nothing. The reason for that is the fact that Nebula was in use, well the device thinks it still is.
There ARE ways to factory reset the device but you either have to
Do it via Nebula, duh!
Use the Windows Only (!!!) ZON Utility. This only works as long as you know the password. Well I seem to do, but I couldn't get ZON to run in wine...
The WebGUI. Well that only works if the device is not managed via Nebula.
This is the crappiest one per se. You can reset the device as long as you have a Zyxel Switch with the so called "Ethernet Neighbor" feature. Dear companies. Stop doing that kind of proprietary crap that hardly anyone can ever use.
So I'm kind of fucked, but am I? There are fallback methods, right? Well the only one that should officially work is flashing via TFTP. The problem with that is that it wouldn't let me do it trying 25.05.5 and 24.10.rc5 but both complained at the atns command.
At least my serial console was working so I just watched the device boot stock firmware and here comes the only positive surprise to me. The device has a fully fledged command line interface, fully documented. Weirdly I found it on a foreign domain. https://www.studerus.ch/support/download/205955
TLDR;
So if any of you guys runs into the very same problem, having the password, but the device still being dominated by Nebula, just:
have console cable connected (only TX, RX and GND)
boot up the device
login with (admin/1234)
configure terminal
apply /conf/system-default.conf
reboot
enter
Then you will have a factory default NWA55AXE and you might still want to buy a Zyxel next time in contrast to me. OpenWrt was then easily flashable via the integrated WebGUI.
I understand (and share) your frustration, having just moved an NWA50AX to OpenWrt. Even with a reset button, all it seems to do is temporarily reset the device, then it asks again for the Nebula credentials. And of course, the Nebula app requires you to register.
I'm not sure why your rant is so lengthy, though - the OpenWrt commit message details all that beautifully. I whipped out my serial cable quickly enough after finding neither the reset button nor the Nebula route were going to work.