Hey folks – wanted to jump back in to let you know that the Particle team is watching this thread closely and is actively working to triage these symptoms into multiple root cause issues with the help of AT&T and u-blox.
In the meantime, if you are experiencing this issue we'd love to have your help capturing cloud debug logs that we will use to categorize and resolve these issues. You can find instructions for doing so here:
We will be a little slower to respond than usual due to the July 4 holiday, but want to reassure everyone that this is a top priority for our team. We're committed to continuing to improve the reliability of LTE M1 connectivity and look forward to squashing some nasty 
s with your help over the upcoming weeks.