Hello Roberlf and welcome to the forum. There is a lot of great information here on this forum.
There are a couple of things you might want to check.
When using cable, you must turn off the antenna booster.
When using cable, you must program your TV to look for a cable input signal.
Then you must do an auto channel search.
There is also an outside chance that the cables in the UDC are switched.
It would help if we knew what model coach and TV you have.
Peace
Dave