I don't think you're doing anything wrong. When the LED on the front lights up green, the thing should actually already be charging. The problem is probably simply that the bank is too big or the solar panel too small to really move anything within a reasonable period of time. I'm not sure if I've ever moved a bar with the solar charging function on my power bank (I've had the thing for almost two years now).

According to the manufacturer, the solar panel on the bank has an output of 2W. So the 88.8Wh would be full in 44 hours. With four total bars, two days of full sun should not be too far from the next bar -- theoretically. But the 2W charging power was probably measured in a laboratory somewhere under ideal conditions. In practice, the value will probably be significantly lower.

The solar panel is no good, it's just too small. I had mine in the sun for 5 days in the best weather and nothing charged. The theoretical 2W are a drop in the ocean compared to the large battery. When I connected the powerbank to a Poertraveler solar panel Falcon 40W, it charged. As a pure powerbank without solar it is very good and has quite a lot of power, you can charge a smartphone several times.

I think that the solar panel is not the best.

The problem arises from the rather large capacity (24,000mAh) and the very small solar panel. If I leave it outside in good weather for about a week and don't charge anything with it, it is full. Maybe it was "only" 5 days.

I have the same problem, mine has also been in the sun for 2 days and I only ever have one light on the power button? Did you return it or does it still work?