Diagnosis
Your wireless devices are likely connecting to the node on the slower 2.4GHz band instead of the faster 5GHz band. The nodes themselves use a dedicated 5GHz channel to talk to the router, but your phone/laptop might not be using the best band.
How to Fix It
Log into the router's web interface (router.asus.com). Go to Wireless and then Professional tab. Look for the setting called Roaming assistant. Set the RSSI threshold to something like -70 dBm. This gently kicks a clinging device off a weak signal, encouraging it to hop to the stronger node or the better band.
If That Didn't Work
Check if Smart Connect is on (it blends the 2.4 and 5GHz SSIDs). Sometimes it makes poor choices. You can turn Smart Connect off in the same Wireless menu to create separate networks (like MyWiFi_2G and MyWiFi_5G), then manually connect your device to the 5G one.
Heads Up
The speed you get on a node also depends heavily on that node's own wireless connection back to the main router. If that backhaul link is weak, everything connected to that node will be slow.