Came across your post trying to trouble shot a similar problem.
I’ve got the pixel streaming all setup on a PC on my network.
Using chrome on my mobile, if I connect to the same wifi network and enter the local IP all good, works really well but if I turn off the wifi and connect using 4G, entering my public IP it still connects, I press the “play button” and all thought the status icon is green the view port remains blank/black.
For what it’s worth, I’ve got this vague notion its something to do with the stun server but on triple checking my settings can’t see anything wrong so maybe not.