Hi,
Here’s the [https://www.electronica-pt.com/esquema/tv/panasonic-esquemas-tv/panasonic-tx-55dx650b-chassis-la64-66216/|service manual] for the TV.
Download and open the manual and then go to p.23 and check if you can access the ''Self Check'' test as described in the manual to find out what’s wrong.
If no good, on p.24 there is the ''Power LED blinking timing chart''. Count the number of times that the TV power LED is blinking to find out which board is the most likely cause of the problem. The TV power LED is the one you said “… red light start flickering…”. The flickering is actually pulses displayed in a set pattern which repeats after a pause e.g. 7 blinks - pause - 7 blinks - pause etc