There are a lot of network related edge-cases that introduces strange problems that locks the modem. I did solve this one, but can't find my answer on the forum. Glad you figured out a way too.
The GSM shield hardware, software and the Quectel module itself, has some severe problems for real world projects... I'm thinking of re-writing the needed AT-commands in a very simple C style with individual timeouts for each command. Our latest discovery were that the Quectel module might take several minutes of trial and error to find a network it can use. A two minute timeout was not enough when the first network refuses it...