System no longer connects to wifi network after router firmware upgrade, how to troubleshoot more?

[Alan Tu <8libra gmail com>]

Hello, A router firmware upgrade has caused my network-manager system not to be able to connect to the 
fastest wifi access point. I'd like to know how to troubleshoot exactly what is going on so I can provide 
feedback to either the NIC driver maintainers, the software stack, the Asus firmware people, etc.

I'm running Debian testing (v11 + updates.) network-manager is at version 1.30.6-1. The NIC is an Asus USB 
dongle running the rtl8814au module from aircrack-ng. The access point is an Asus RT-AX88u router (irony, I 
know).

This system was connected to a wifi network called wln. Because of old smart speakers, I had to split my 
network. wln runs on the 5 GHz band and "wln_legacy" runs on the 2.4 GHz band. The server was connected to 
the wln SSID.

I upgraded the Asus router firmware, a mistake in hindsight. My Windows tablet and my iDevices are talking 
with the 5 GHz wln network just fine as before. But network-manager (or something underneath that) is not 
able to connect.

The wln network is still visible:
$ nmcli dev wifi list
...
*       04:D9:F5:FA:D4:F8  wln_legacy  Infra  7     540 Mbit/s  87      ▂▄▆█  WPA2     
        04:D9:F5:FA:D4:FC  wln           Infra  52    540 Mbit/s  24      ▂___  WPA2      

But when trying to bring the wln connection up, I see messages in the system journal like this:

Sep 03 12:46:11 raptor NetworkManager[555]: <info>  [1630698371.4330] device (wlx1831bf53a3a2): Activation: 
starting connection 'wln' (afc892b2-7e5f-45a5-9858-16a155bc8bb3)
Sep 03 12:46:11 raptor NetworkManager[555]: <info>  [1630698371.4384] device (wlx1831bf53a3a2): state change: 
disconnected -> prepare (reason 'none', sys-iface-state: 'managed')
Sep 03 12:46:11 raptor NetworkManager[555]: <info>  [1630698371.5565] device (wlx1831bf53a3a2): state change: 
prepare -> config (reason 'none', sys-iface-state: 'managed')
Sep 03 12:46:11 raptor NetworkManager[555]: <info>  [1630698371.5575] device (wlx1831bf53a3a2): state change: 
config -> need-auth (reason 'none', sys-iface-state: 'managed')
Sep 03 12:46:11 raptor NetworkManager[555]: <info>  [1630698371.5637] device (wlx1831bf53a3a2): state change: 
prepare -> config (reason 'none', sys-iface-state: 'managed')
Sep 03 12:46:11 raptor NetworkManager[555]: <info>  [1630698371.5751] device (wlx1831bf53a3a2): supplicant 
interface state: disconnected -> scanning
Sep 03 12:46:37 raptor wpa_supplicant[564]: wlx1831bf53a3a2: Reject scan trigger since one is already pending
Sep 03 12:46:37 raptor NetworkManager[555]: <info>  [1630698397.1961] device (wlx1831bf53a3a2): state change: 
config -> failed (reason 'ssid-not-found', sys-iface-state: 'managed')

How can I gather more debug information, to better articulate what is going on? I'd appreciate some pointers! 
Thank you in advance.

Alan