Hi Neil,
I have a number of these devices in the field and am also looking into some issues with pppd.
I am really curious whether you discovered the root cause or have any additional information? It may be useful for me as well.
One thing that put me on the wrong foot a while ago is the band setting. If you haven’t checked, I would recommend you verify AT+WMBS?
With kind regards,
Maarten van Berkel