Whatever comes with should work mostly fine, whitelist unlocks are for those didn’t option a WWAN. All WWAN modems are same, usually they have 3-5 serial ports for modem control, data, debug, and NMEA. My hunch is your ttyS2 takes “GPS enable” command and it’ll start murmuring it on ttyS4.
Refs:
https://whirlpool.net.au/wiki/sierra_advanced_gps
https://forum.openwrt.org/t/problem-accessing-gps-data-from-...
https://gitlab.freedesktop.org/mobile-broadband/ModemManager...