That's a known "feature" of some True RMS DMM, any DC offset will make the measure more or less useless.
Other DMMs might give only the AC RMS value, and still others have a true AC+DC RMS mode (not what you are looking for).
I think your UNI-T falls in the first category...as does mine (61D).
My Fluke 87V*, instead, should ignore the DC offset and give just the AC RMS.
They internally use the same True RMS converter (AD737), the difference is in the way it's coupled to the inputs.
See, for example,
this tread, where more complete explanations are given, and other UNI-T models are also discussed.
A large enough capacitor in series will allow you to read the ripple RMS value by blocking the DC component.
*Brag mode on: "Yes, my other DMM is a Fluke"