I successfully get ubnt radio mode by function:
string_substring(oid("iso.org.dod.internet.private.enterprises.ubnt.ubntMIB.ubntAirMAX.ubntRadioTable.ubntRadioEntry.ubntRadioMode.1"),0,2)
For AP I get "ap" and for STATION I get "st"

Problem:
string_compare(string_substring(oid("iso.org.dod.internet.private.enterprises.ubnt.ubntMIB.ubntAirMAX.ubntRadioTable.ubntRadioEntry.ubntRadioMode.1"),0,2),"st") always returns 2 nevertheless the radio mode.
For "st" should be 0, for "ap" - 2.

The Dude shows no scripting errors.
Where do I make a mistake?