Try this:
1. Connect to a none 4K TV, play something, than go back to a 4K TV set!
or
2. Set the video output to 4K/30 or 4K/24 Hz fixed (not automatic), turn off deep colour! On your TV/AVR be sure you connected the Pro 4K to a HDMI2.0 port (sometimes there are 1.4 and 2.0 ports too on the same device), and set "Enhanced" option for the given HDMI input, if there is such option on your TV/AVR!
It is not a hardware problem but a HDMI handshake only.