Do you know what kind of Sprint SIM you are using? Depending on whether your device is an LTE or LTEA version effects what type of SIM is compatible. If you aren’t using a compatible SIM that may be the source of your problem.
LTE devices are compatible SIMGLW106R SIMS
LTEA devices are compatible with SIMGLW106Q SIMS
Below is a forum post discussing the different types of Sprint SIMs that should be used for Peplink products.