In addition to what Sara said (she’s the best!) about the UUIDs for the station when I’m doing that many stations at once I easily also miss an occasional apn = “hologram”.
If the code all checks out, I second the experience that it’s strange how sometimes it takes way longer for one or two modems to connect, even when things are from the same batch. I often let them run for a lunch time and come back to see if they connected.
And if I think it all should be right, I shut it all down and reseat all of the modem and battery connectors (not the coin battery). Then I scrutinize the Hologram data for each card, confirm that it’s the one on the station, etc.