I do not know why main poster is challenged this much about his/her
reasonable expectations in DMM capabilities. He/she simply asks for DMM to work well with very simple and standard waveforms, nothing fancy or extreme. And crappy UNI-T DMM simply fails to deliver. TRMS AC only or TRMS AC+DC discussion has nothing to do with this at all.
I agree that UNI-T brand is not the best and very low end, but this case (AC measurement with DC offset) DMM could perform much better, really. This and other case, where UNI-T choose to save penny and make DMM much worse for no good reason, do no put any trust for UNI-T branded DMMs.
Have you seen any equipment repair instructional videos from, say, 80s and 90s?
Many of these videos included segments where section with linear regulator powered by rectified AC is checked with DMM. DMM is put in DC mode, probes are put on main capacitor before linear regulator to check DC level. Then meter is changed to AC mode to check for ripple (with 50/60Hz or 100/120Hz frequency). Very typical and straightforward operation, no high frequencies are involved, simply DC or AC at 50/60/100/120Hz only.
And UNI-T UT139C falls on it's face right here, in very simple situation! Fail
In the field, one typically does not know in advance what kind of voltages are present on the lines. That is why one brings DMM - to get information about voltages and frequencies.
In OP case, he/she
knew in advance that DC had low frequency AC ripple, yet could not measure it properly! What about cases where he/she does not know what to expect in advance? One has to
trust equipment to show reality, not some nonsense numbers. UNI-T meters clearly lack this trust for a reason.