Good morning. This morning, for unknown reasons, the router stopped working (I should mention that it had been working properly for almost 1.5 years). The router boots from SD cards. After the failure, the red LED blinks very gently (almost invisible) at short intervals. After unplugging and reconnecting the power, the red and blue LEDs blink rapidly, and then, as I mentioned, only the red LED blinks gently. The RJ-45 port is lit continuously. Could you please tell me if this means the router is dead?
Hi,as always without uart output it is nearly impossible to find rootcause 
please connect debug uart, poweron and post bootlog here
I have never had any contact with Uart and unfortunately I do not even have the appropriate equipment.
Then now you have a good reason to start 
I would recommend a ft232rl, set to 3.3V.
It is not very difficult.
Settings are same as on R2,but for R3/R4 better use ftdi as eric suggests.
first step for me is allways changing the power supply and check.
You’re right, I’ll replace the power supply first.