Russian armed forces are currently advancing at 15 to 20 kilometres per day

That was true for well fortified positions on defensible terrain. Dnepropetrovsk region doesn't have built up defenses and it's open plain.

The claim of 15-20 kilometers per day would represent an extraordinarily fast advance rate that doesn't match current battlefield realities. The actual advances are measured in hundreds of meters per day or small numbers of square kilometers per month, not kilometers per day.