In a pickle update:
Long version:
Well I've tried everything I can think of to fix the sketchy cellular data connection on my shiny Cosmo.
I've checked IMEI numbers against the numbers on the box, checked they weren't blacklisted online, checked APN settings for consistency, build numbers, image parts, etc, every setting I can see looks okay.
I've repartitioned and reflashed to Android or Linux, reinstalled Debian and Ubuntu Touch, unlocked the bootloader via FastBoot, started again, and again, but still no reliable network connection on my new Cosmo, whereas the old damaged one still works after the same routines.
I've tried multiple sims in both slots, with or without the sd card present, on all operating systems, but still no joy on my newer Cosmo.
I tried going back to Android V19 firmware but I couldn't get recovery mode to see the installer on my sd card. Probably wouldn't have done any good.
I haven't tried the SP FlashTool yet. I haven't read up enough on it yet, haven't found the relevant files, I'm not sure how relevant this tool still is, and I'm out of my depth.
I've done everything I dare to get busted old Cosmo and shiny new Cosmo back to identical settings to satisfy myself that this is not a software or network issue but a hardware issue, but for a couple of reasons I'm still not sure.
After a 'Factory Reset', reflashing via recovery mode, boot options on old cosmo vs new cosmo are still different? That doesn't sound like a reset to me. I would expect behaviour to be consistent, (if annoying), on any device after a Factory Reset. Is there a more fundamental level of reset available?
In Ubuntu Touch I've also noticed some weird behaviour when I try to make a call or send a text. On occasion the messaging or call app brings up the message 'No sim card, please insert a sim card and try again', whilst at the same moment I can open the pull down menu and see my sim card sitting in either slot, recognised and displaying 2G or 3G. Surely that's got to be software, (or firmware or settings), but not hardware. (See attached screenshot)
On Android I'm also getting some strange behaviour where if the network connection is showing off, and I try to turn it on, a popup asks me if I'm sure I want to turn it off, when it's already off.
Next stop a report and email to Planet to see if they can help, before I'm tempted to open the bonnet and lose half the fixings down the back of the couch.
Short version:
To be continued.