I recently bought the same Asus XT8 set and had some problems at first. Since updating the routers firmware to the latest beta, I had no problems anymore. I left WiFi 6 and Smart Connect on, so I have 1 combined SSID for all WiFi bands. Works flawlessly now!
I even activated the second 5GHz band for connecting clients since I wired both nodes.
Changelog of the XT8 beta firmware says it solved issues with IoT compatibility.
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