FONA 3G, batteries, polarity and dodgy serial connections

I have an old Fona 3G that I want to get working. I connected via USB from a windows 10 PC after finding some drivers - but the serial connection comes and goes (always on the same com ports though - 3 of them, not 4.). Anyone any ideas?

The adafruit documentation says it requires a battery so I bought a battery 1200mAh 3.7V Li ion, but that hasn’t helped. I’ve left it on to see if it just needs charging, but also, the manuals says that the adafruit batteries have the right polarity and non-adafruit batteries might have a different polarity. How would I test or correct that?

It has a SIM card, but of course might be broken - it’s been sitting about, sealed in its wrapper, for the last five years or so.

EDIT: Well I pulled the pins out of the JST PH plug and swapped them around, and the serial connection is now stable, so that was the problem, it seems. I can’t believe it - I looked at various pre-wired JST PH sets on a popular website and many of the comments were along the lines of ‘the polarity is wrong, I burned out such and such’. Anyway, I’ll leave this here in case it’s useful.