Thanks for posting to the Community Forum. That is correct for Class A devices a downlink can only be communicated to the device upon receipt of an uplink. Once the device transmits there are two receive windows Rx1/Rx2 at 1/2s respectively that the device will listen for a downlink. We see that you have created a Senet Portal account but there are yet to be any devices added to the account.
The portal documentation has a detailed explanation of how to register a device through the portal. In addition, there are configuration guides on how to configure the Notification Target to forward data to your back-end as well as configuring the downlink API. Please find the links to these documented workflows and let us know if you have additional questions.