The receptor on OpenSprinkler DC has a 2.5mm diameter center pin. The cable you bought shows its internal diameter is 2.1mm which explains why it won’t plug in. Could you find another one that has 2.5mm internal diameter?
Admittedly 2.1mm is more common. On this batch of the OpenSprinkler DC, the factory mistakenly used 2.5mm. The power adapter included in the controller kit fits both 2.1mm and 2.5mm so it’s ok. However, if you are trying to find your own cable or adapter, 2.5mm happens to be the less common one.
In the worst case, you could cut off the male plug connector from the power adapter as well as your cable, and basically ‘switch’ them, so that the plug connector from the adapter goes to the end of the extension cable, and vice versa.