thingsHub allow to easily send downlinks to devices regardless of which network server are used. 

curl -X PUT ''
     -H 'Authorization: Bearer <YOUR_TOKEN>'
     -d '{"confirmed":false,"data":"string","port":0}'

The field confirmed indicates if the downlink should be confirmed or not. Note that it depends on the LNS on what exactly downlink confirmation means. Confirmation might refer to the sending of the downlink or to the reception of the downlink on the device (with the next uplink).