how to avg function in labels in maps

[hashbangtech]

hello,
I want to use avg function in map labels. Currently using last function
RSSI:{?last(/AZA-1/WirelessRegistrationTable-Signal[6C 3B 6B 46 23 8F ])}
RSSI:{?avg(/AZA-1/WirelessRegistrationTable-Signal[6C 3B 6B 46 23 8F ],5m)} is not working.
Kindly helpp
ty