Heltec Wifi Lora 32 V2 868~915 instead of 868 on package


I have bought 2 of the heltec wifi lora 32 V2 devices (868 MHz). i wrote some code and it dit the job perfect so i bought 25 extra devices. now i get the same devices but on the package it says 868~915.
so i thougt it was no big deal because i need 868 but when i upload the same code on every device and 50% of the 25 devices does the job and the other 50% does it not. so its not the problem in the code but in the factory settings of the module. how can i resolve this problem. note: i do not use the heltec library but the lmic library.