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.
Are you able to exchange at an Xfinity retail store?
My Flex has been working for several months. I'm connected to my TV through an HDMI switch. When I first power up the TV at night, it shows a message that it's trying to load configuration from the HDMI port and that goes on for a while and finally gives up. My solution is to briefly switch the HDMI switch to another channel and then back to the Flex. This seems to wake up the HDMI port on the Flex. I'm betting the Flex keeps its HDMI port in a power-saving mode and fails to negotiate a proper power startup with the TV or switch. I have a similar problem with my old video card in my PC and my LG TV I use for its display. I have to power up the TV first and then hit a key on my PC to get it to connect. Any other order and it fails. So the HDMI standard is having the same problems we saw with USB when it first came out.