This modem was provisioned with Generic configuration (PRI)
you need to upgrade the FW to Sprint:
https://source.sierrawireless.com/resources/airprime/software/airprime-em73xx_mc73xx-fw-package-latest-release/
Sprint normally uses “otasn” on PDN1 but is is only used by Sprint for over the air provisioning.
Data “n.ispsn” is on PDN3. FW upgrade should take care of that.