I have the same issue on 388.1. It looks like the hyperlink to upload the firmware is nonexistent. You click the button and nothing happens because all it is is the Upload button with an underline. Factory reset and using other devices won’t resolve the issue because there is no link for it. I don’t want to take my network offline for to long to figure this out.
I have two ways to try and fix it.
First is if someone can post the url that comes up in the pop up window for firmware upload. It may allow for you to go to that address manually in another window and upload the firmware. Hopefully fixing the issue going forward. As far as I can tell, there is just no way to actually get to the upload window.
I haven’t tried it yet, but you may be able to swap out the main access point and add it to another asus network as a node. The firmware upload may work that way since it uses a different window / method. I haven’t tried this yet because the last time I was messing around with it, my router configuration got corrupted. I made a save right before doing it, but once I uploaded the saved configuration, it immediatly corrupted again. I had to restore to a previous version that was far older than I would have liked… Anyway, be prepared to use an older configuration file if you do mess around with it like this. I have a feeling this issue is stemming from some sort of corruption in the current firmware and any recent saves will not fix it. Also it won’t upload after factory reset either.
If you’re able to resolve it with either method, let me know. Also, as far as I could find, there was no reliable way to SSH in to do the update.