An UTH is not meant to warm the tank. They are for the hotspot and do very little to heat the rest of the cage.....as you found out.
How long have you been using the CHE like this to test it? Is the snake it there? A CHE takes a good while to level out and show you what it will really gives you.
An issue when you have to move the heat fixture from the screen lid is you take the heat away from the cage whenever you enter it by removing the lid. This was a big reason I used the stands. It let me leave the CHE in place so the heat is still pumping into the tank instead of cooling down. This made my temp checks more accurate by reducing cooling to the hotspot I was creating.
Also not having to move the hot fixture is much safer and keeps a person from the dangers of placing the heated CHE to something that can melt and catch fire. A CHE gets BLAZING HOT.
What I did to help my tank in my cold apartment was cover the back and sides with thick art paper. This kept more heat it and raised my temps by itself. Then I also made a cardboard cutout for the screen section of the lid. I left a window for the heat from the CHE to pass through and covered it with foil. This kept even more heat it and helped keep some humidity.