From my experience, the following way of configuration will always fail.
Condition 1. Asus AX88U as the top router (RT-AX88U_3004_388.4_beta3.zip).
Condition 2. AX55 as Aimesh node (ASUS RT-AX55 Firmware version 3.0.0.4.386.51598).
AX55
1. From page Administration - Operation Mode
2. Switch to AiMesh Node from fully configured Wireless router Mode(default)
3. Automatic reboot
4. Without powering off even once.
AX88u
1. From page Aimesh - Add AiMesh Node
2. Search
3. AX55 will be discovered( with Wifi mark)
4. It will fail at about 35%
5. Showing unable to add Aimesh.
Following this path it fails every time.
RESET BOTH ROUTERS TO DEFAULT.
To break this ice, after switching to AiMesh Node.
You need to do,
1. Power on AX55
2. Wait until it is fully booted
3. Check if WebUI is accessible.
4. Power off
5. Do 1 to 4 power on/off at least twice.
6. Power on and wait WebUI to be up
7. Connect AX88U LAN to AX55 WAN
8. AX88U From page Aimesh - Add AiMesh Node
9. AX55 will be discovered( with ETH mark)
10. Progress bar will exceed 40% , and pairing will be a success
**END NOTES
AX88U updated to latest merlin via laptop w/ webui. (RT-AX88U_3004_388.4_0.zip)
AX55 updated to latest stock via laptop w/ Recovery App. (ASUS RT-AX55 Firmware version 3.0.0.4.386_51948)
Reset both devices to defaults. (BOTH DEVICES ARE NOT CONNECTED TO ANYTHING)
Set laptop to 192.168.50.2 and connect to AX55, check webui works.
Power down AX55 and power on again to check webui.
Plug ethernet cable into LAN AX88U -> WAN AX55. (can this be skipped?)
add node via AX88U webui. Detects and adding is successful. (detected node as WIFI).