Try connecting the mini HDMI to HDMI cable from the Pi to the screen first, then turn on the Pi after. I have the same setup and it displays the startup prompt for me every time I connect the Pi to a screen. My screen is a regular computer monitor though, but I think that shouldn’t make a difference, even if you connect to a TV.

Another common problem with the Pi is the ssd enclosure. Some of them don’t work so well, so look online and look for a good one recommended by Umbrel. Initially, I used an old ssd enclosure sitting at home and the Pi just wouldn’t mount the drive. I then bought an Orico enclosure as recommended by Umbrel’s site, and it worked.

Also, make sure you are using a genuine Raspberry Pi power supply. The aftermarket power supplies could sometimes not supply reliable enough power and cause issues with the hard drive mounting or other issues.

All in all, Raspberry Pi is a good low cost starting point for running your own bitcoin node and lightning node; but it’s not the best hardware out there. You’re better off installing Umbrel into an old computer, which has much more reliable hardware, and run your nodes from there. Especially if you ever start to put a decent amount of coin into your lightning node, you wouldn’t want it running on a Raspberry Pi.