RT-AC5300 cannot upload FW. Help please.

My setup:

ISP: Virgin Media 150mbps
Router: GT-AC5300 (main) + RT-AC5300 (mesh node)
Modem: VM Hub (modem-mode)
AiMesh Node Connection Quality: -60 dBm

I want to load the Merlin FW to my RT mesh node but I'm having some difficulties. The steps I have taken are:
Login to GT WebUI (192.168.1.1)
Click on Navigate
Click on Firmware Upgrade
Under AiMesh Node click on 'Upload' where it says "Manual Firmware Update"

This opens a new window...

RT-AC5300 cannot upload FW. Help please.