@David-TP 

So, yeah, I've gone through the bricked firmware method 2 process several times of reflashing the firmware via the direct connection from my laptop to the ethernet port.  It flashes just fine...goes to 100% and tells me to wait until the router reboots.  Then I never see any indication that the unit is rebooting.  The LED never blinks or anything...just stares at me with its evil yellow eye.  Last time I left it sitting there for 5 minutes before I power cycled it.  I also tried downgrading the firmware, but that didn't do anything different.  Pressing the reset button has no effect whatsoever except it does make the LED go out like it should just before flashing the firmware.  Doesn't matter if I momentarily press the reset button, 1 second, 10 seconds, or hold it in for a full minute...nothing changes.

I've double checked that I'm using the correct firmware...Model Deco M4R(US) v2.0

I've had it since about January.  It was set up with three M4R and one M3W using gigabit ethernet backhaul between the M4R units.  Just basic setup with a main and guest wifi channel and no port forwarding setups or address reservations (although I do need to set that up for my cameras).  The main unit is the one that has stopped working.  I was able to get all the others restarted by hitting the reset button and running the setup routine.

It isn't a power supply issue, as I've tried several.  The unit was on a protected outlet on a UPS, so I doubt there was a surge or brownout issue (happens frequently to electronics where I live).  There wasn't any unusual activity at the time of the failure as I was the only one on the internet at the time and all I was doing was watching a youtube video as I worked out on my spinning bike.  The ISP provided cable modem did lose its IP and I couldn't ping the cable modem at all using the normal 192.168.100.1 address, and eventually I had to call the ISP to get them to reset the cable modem before I finally was able to get internet working again.  Power cycling the cable modem would give all the correct lights on the modem panel but it still wouldn't pass an IP to the Deco unit.  Finally I pulled my old Archer C7 out of a cabinet and hooked it up to verify that the cable modem wasn't working correctly and also that the Deco unit had also failed.

So after all is said and done, now I have working internet again and a working mesh with the two M4R and one M3W, but I have a gap in my kitchen area that I need to fill with this inoperative M4R.

Since the firmware flash process seems to be working, then it seems there should be a way to see what's going on using some ancient command line tool.

I actually just hit the setup process again just to see if it would set up without the blinking blue LED.  Of course it didn't.  Why would I think that would work?  It just stares at me with its jaundiced little yellow gorgon eye.

  @David-TP 

Would it be possible to SSH into the router since I can, at least, get to the firmware flash page?