OK - so I cheated and reflashed the FIN board with BalenaOS, just to see if the modem/SIM card was recognized.
It was.
---------------------------------------------------
root@18f3bf2:/resin-boot/system-connections# mmcli -m 0
  --------------------------------
  General  |            dbus path: /org/freedesktop/ModemManager1/Modem/0
           |            device id: 25564e2468d1e70306b433bf880c542656d7795d
  --------------------------------
  Hardware |         manufacturer: QUALCOMM INCORPORATED
           |                model: QUECTEL Mobile Broadband Module
           |             revision: EC25AFAR05A04M4G
           |         h/w revision: 10000
           |            supported: gsm-umts, lte
           |              current: gsm-umts, lte
           |         equipment id: 861641041224386
  --------------------------------
  System   |               device: /sys/devices/platform/soc/3f980000.usb/usb1/1-1/1-1.2
           |              drivers: option1, qmi_wwan
           |               plugin: Quectel
           |         primary port: cdc-wdm0
           |                ports: ttyUSB0 (qcdm), ttyUSB2 (at), cdc-wdm0 (qmi), wwan0 (net), 
           |                       ttyUSB3 (at)
  --------------------------------
  Status   |                 lock: sim-pin2
           |       unlock retries: sim-pin (3), sim-pin2 (10), sim-puk (10), sim-puk2 (10)
           |                state: enabled
           |          power state: on
           |       signal quality: 0% (cached)
  --------------------------------
  Modes    |            supported: allowed: 3g; preferred: none
           |                       allowed: 4g; preferred: none
           |                       allowed: 3g, 4g; preferred: 3g
           |                       allowed: 3g, 4g; preferred: 4g
           |              current: allowed: 3g, 4g; preferred: 4g
  --------------------------------
  Bands    |            supported: utran-4, utran-5, utran-2, eutran-2, eutran-4, eutran-12
           |              current: utran-4, utran-5, utran-2, eutran-2, eutran-4, eutran-12
  --------------------------------
  IP       |            supported: ipv4, ipv6, ipv4v6
  --------------------------------
  3GPP     |                 imei: 861641041224386
  --------------------------------
  3GPP EPS | ue mode of operation: csps-1
  --------------------------------
  SIM      |            dbus path: /org/freedesktop/ModemManager1/SIM/0
---------------------------------------------------------------
So, the card and the socket are probably OK.
Thoughts?
John
