Can I use an ASUS RP-AC1900 as just a plain access point?

The situation: three floor house. Not big, but enough stuff in the way that signal is not great in several rooms. The main router/access point is on the second floor. It's an RT-AC88U which I've been super happy with, so when my company gave a little bit of extra money to help support remote work, I sprung for a couple of RP-AC1900 extenders without doing much research.

But not, like zero research! The brochure page clearly says:

• Range extender mode
• Access point mode
• Media bridge mode

... so I thought, okay, I'll try this fancy AiMesh thing and see if it works, and if not, I'll go to plain old access point mode. I have ethernet from the second floor office to a computer location on the top floor and by the TV on the first floor, so it all seemed perfect.

And the first-floor node does seem fine. Plugged it in, went to the router web interface and told it to find the AiMesh node, everything's up, and seems like connectivity is better. I noticed that it's using overlapping channels with the main wifi, which seems weird, but it wasn't causing any problems.

Then I added the upstairs one, and ... not so good. Definite performance degradation, drops in video calls worse than before, etc.

So, I thought, fine, I'll just reconfigure as a regular access point, set up my own channels, even cringe and set up different network names for each floor like a cave-person if that's what it takes.

But now I'm frustratingly stuck. I can easily remove the AiMesh node from the web interface, but how in the world do I reconfigure it? I expected to be able to do the normal thing and configure it via a web interface running on the device itself, but that doesn't seem to be running (even after a factory reset). There's an Android app to download, but that seems to just find my existing main router. There's a separate app for setting up an Extender, but I don't want to set up an extender. What can I do?

Kind of annoyed with myself because I already threw away the packaging. Hopefully there's some secret I'm missing?

Aha! It's actually doable. The key is: I was connected to the WAN port — the connection to my main router. When the RP-AC1900 is unconfigured, the device won't respond on that port. I had to connect to the LAN port, and then point my web browser to http://192.168.50.1 (note 50 — not sure if this is random or for all of these devices) and then follow a brief wizard to enable access point mode. (You can also connect to an open wifi network provided by the unconfigured AP... I hadn't noticed that before.)

Once that's done, you can reconnect the WAN port. The AP will get an address assigned on your normal network by the main AP (or you can configure manually) and you can connect to the web interface there to finish configuration.