I would say it's finicky. Mine took several tries for it to eventually stick. Eventually I was able to get it to stick at 320Mhz. I don't believe I have any interference around either. All of mine said SP mode available as well.
Still keeping an eye on mine. I just noticed mine went back to low power mode.
Edi: It appears that leaving it on 20/40/80/160/320hmz is working best for me. Mine is currently using 160Mhz. I didn't play around with using different channels, just letting the router decide at the moment.
Wonder if Android vs iOS may be different... Also I believe the ASUS Router App asked to change the Location permission, which I did not mind allowing "all the time" (EDIT) and "precise location".