Hi there, I recently went through this exact same issue. It is a common frustration with the Pro Box. Before you give up on it, here is a 4-step checklist that usually helps:
Check the HD and 4K Add-on: Ensure the HD and 4K add-on (usually 6 pounds a month) is actually active in your account. Without it, the box defaults to Standard Definition for internet channels, which looks very poor on a 4K TV.
The 1080p Trick: Many find the Pro Box is poor at upscaling. Go to Settings, Picture and Sound, HDMI Display and try changing it from UHD to 1080p. Often, a Sony or LG TV does a much better job of upscaling the signal than the box does.
HDMI Enhanced Mode: Check your TV settings for the HDMI port the box is using. Look for a setting called Enhanced Format or HDMI UHD Color. If this is off, the colors can look washed out or milky.
Internet Mode vs Aerial: If you are using the box without an aerial, the quality depends on your Wi-Fi. If the Wi-Fi dips, the box lowers the resolution. If you have an aerial, try using that instead for a more stable picture.
Hope this helps! Using the apps directly on your TV or a console like the Xbox or PS5 often gives a much better picture than the box itself.
