I don't have a SDS2504X Plus, but used a SDS2102X Plus with a 20Hz 1mv peak to peak at the input. The input channel setting is 500uv/div, DC coupled, 20MHZ BW limit with 1M Impedance, the trigger was set to Normal, Edge, Rising, HF Reject, Level 0V and Noise Reject ON. Used the standard scope probe at 10X and triggering is OK, wanders slightly and counter reads 19.99~20.02Hz. The scope actually triggers quite well considering the small input, and the counter does a respectable job of counting.
Here's a screen capture, hope this helps.
Edit: Added FFT screen capture as well.
Best,