I am having the same issue. Received my new flex but remote light will only flash red and tv doesn't recognize the box is connected, despite restarting multiple times.
Are you sure you're on the correct HDMI input? So to clarify your issue, your getting a no signal message on your TV b/c the flexbox is not being recognized by your TV?
I too have this issue. Though mines a little different. I connected everything with the cables it came with, all new. Connected it to a HDMI 2.0 for the 4k resolution. I know the port works because my Playstation 4 Pro was connected to it.
When I powered it on the first time I saw the "Welcome" screen for like 2 seconds and then I get "signal lost" from my TV screen. Then a few seconds later the "Welcome" screen comes back, then "signal lost again" and back and forth, over and over.
I don't understand what it's doing and I'm one of those techy kind of people everyone else asks for help from. I figure the box is just bad, maybe comcast had these Flex boxes made cheap and they are all garbage. Does anyone know if the Xfinity Stores are open and accepting returns on these wonderfully reliable devices? I'd like to get a replacement and see if that works, but I'm already discouraged and don't expect any luck. And really the only reason I wanted this thing is for Peacock so I could watch the "Psych 2" movie on USA when it comes out.