I think the external monitor settings
I had the same problem there is a setting I don't remember now but I will turn the screen back on and let you know soon if I find time
I had an external recording screen and in the main menu there is an option related to the degree, strength or method of receiving the signal and the options were two options made up of numbers, one of which was working with 5Diii
and the other works with 5D iv
Try searching the screen settings in the signal capture