DIAGNOSIS The nodes are likely on different firmware versions, or you're trying to use an incompatible older router as a node.
FIX Update both devices to the exact same, latest official firmware.
- Update your RT-AX86U Pro (main router) first via Administration > Firmware Upgrade.
- Hard reset the node you want to add (press its reset button for 10 seconds).
- Connect the node to the main router via Ethernet cable temporarily.
- In the main router's interface, go to Aimesh > Search. It should now find the wired node.
IF THAT DIDN'T WORK Disable all special wireless settings on the main router before searching. Go to Wireless > Professional and temporarily disable Universal Beamforming and MU-MIMO for both bands. Also, ensure the main router's 2.4GHz and 5GHz SSIDs are set to the same name (Smart Connect can be on).
HEADS UP The RT-AX86U Pro can only act as an AiMesh router, not a node. You cannot add another RT-AX86U Pro as a node to it—you need a model designed to be a node (like an XD series or older AC model).