Hi Mark
No worries for contacting me, I try to give all questions in the MKR1400 and MKR1500 an answer because I am also glad when i get help on other topics. That should be the open source spirit I assume ;=)
TBH I turned away fromMKR1500 to MKR1400 because the global connectivity is better and I need just a small bandwidth so I will answer a bit from past memories..
The
+CEREG: 0,0
means the moblie network is not leting you in.
First step: Please load the sketch NB Scan Networks from MKRNB Examples->Tools and paste the result here.
BTW: IMHO you need the latest available firmware, the soldering part is not that drastic, the heck is more with the windows software and the modem, see :