Once downloading is setup, as soon as you exit the settings, a download will be initiated.
At any time, you can double tap the screen to initiate a download.
Having said that, Android also may throttle the background service (it’s intended to be a power saving measure – and yes its really annoying and nothing can be done about it) so even if you initiate the download it may take some time because Android arbitrarily throttles background services.
When working with Android patience is required. Give it some time, it will run. Once the service starts, then things become responsive.