Diagnosis
A bad firmware update can happen, either due to a buggy release or a corrupted file during upload. The new firmware might have reset or introduced incompatible settings.
How to Fix It
The first step is a hard reset. Use a paperclip to hold the reset button on the back for about 5-10 seconds until the power LED starts blinking. This reverts all settings to factory defaults. Set up the router again from scratch (don't restore a saved config file, as it might bring back the problem). Test performance. If it's still bad, you'll need to manually downgrade the firmware. Download an older, stable version from the ASUS support site. Under Administration > Firmware Upgrade, manually upload that file.
If That Didn't Work
Some firmware updates change the default wireless professional settings. After the reset, go to Wireless > Professional for both bands. For the 5GHz band, ensure "802.11ax / WiFi 6 mode" is enabled. Also, try disabling "Universal Beamforming" and "MU-MIMO" temporarily to see if stability improves.
Heads Up
Always check the ASUS support forum for your specific model before updating firmware. If many users are reporting problems with a new version, wait for a hotfix.