Argh! Yes, that was the issue (it was responding on 50.1.) Thanks so much.
It looks like the current release firmware does not properly support the LEDs on this router; after flashing the 386.4alpha2 firmware the LEDs are acting as they should.
Thanks, I think I'm all good now. Had to...
Just got an RT-AX86S router (because it's hard to find the U version at the moment.)
Immediately flashed Merlin's current (RT-AX86U_386.3_2_cferom_pureubi.w) on the router and after reboot the setup dialog was missing the 'add as AIMesh node' option. Thought that was weird so I held down reset...