The difference in wifi performance will be down to the different wifi radios/chipsets used in the AX89X. It uses QCA radios whereas the RAX200 uses Broadcom radios. Anyway I've gone back to using the RAX200, as I'm not willing to accept worse wifi performance in exchange for using the OpenVPN client. But I will keep the AX89X for a while as a backup router, once Asus upgrade the firmware with better QCA wireless drivers then maybe the AX89X might perform better/on a par with the RAX200.I'm noticing that now too. Also, I'm not sure what setting might be causing it or if it's because the wifi chipset on the iPhone is "more compatible" with it, but with my RAX200, the iPhone would always auto-connect to its 5GHz network once I got to a certain point upstairs. With the AX89X, the iPhone never connects to it automatically, instead keeping its connection to the Ubiquiti Unifi AC Pro on the lower level of the house.
So I have that issue along with the not-fully functioning client list on the AX89X.