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