The Sync tab in the web interface is a one time fire and forget.
To setup a periodic sync you’ll need to setup a cron tab (on the ssh interface) .
Use putty to connect to emonhp.local (or the ip address of your instance)
default username is pi and default password is on the paperwork that came with the unit.
then type
crontab -e
at the bottom of the file add the below line
*/5 * * * * php /opt/emoncms/modules/sync/sync_upload.php
This will upload the data to emoncms every 5 minutes.
Let me know if this helps 