Provisioning the keys in grp2616 template is weird. Weird in the other grandstream templates too. It looks like the pcodes start P1363 for lines 1-6 and P23800 lines 7+.
You might want to manually add a lot of keys through the phone's web server, export the xml to make sure what Pcodes it is using. Then compare that to the device edit page download xml in fusion as a start.
Did you try any other templates like grp26xx?