I have the Asus RT-AX88U (not to be confused with the AC88U). It has the latest firmware offered by Asus for this router.
The Port is connected by Ethernet wire to the router. I can see the Port as a client in the router’s client list.
I disabled “Airtime Fairness” in 2.4 and 5.0 GHz in settings and the Port is not recognized using the Sonos S2 Android app. I am connected to the 2.4 GHz band using the phone. I am correctly logged into the Sonos.
I even turned off QoS, and I still cannot connect to the Port. (I need the QoS for WFH, so shutting it off was just testing to see if the Port will connect without it.)
Note that all other devices have no issues with this router. Sonos Port (and prior to this, the S1 controller) were the ONLY devices that have problems with this router. I am also unwilling to “roll back” the firmware since the current driver patched security issues with the device.