Several things jump out at me.
Near south facing window
Light levels drop off dramatically as you move away from the source, in this case the window.
Radiator under window on low heat
Our homes are very dry and being close to a radiator makes things worse.
Gets watered twice a Week
Never water on a schedule, but according to the needs of the plant in question.
Read through this thread paying particular attention to what we are calling 'the chopstick method'
https://www.helpfulgardener.com/forum/v ... php?t=1479
In general, most trees are going to struggle indoors unless you make a real effort to provide them what they require. Your Chinese Elm will do much better outside once the weather breaks for good.