but I still need to test the reconnect logic etc.
In my experience the nasty points are “partial network outages”, which can’t be tested. Some of such errors tends to be not detected by the modem and therefore are not reported as socket-error.
Some SIM-card provider offer a Web UI to reset the connection for a specific SIM card. That enables to test a lost PDN.
You may also use a separate server in order to stop that. Than enables to test application-connection issues.
A steal-cooking pot may also work. If it’s thick enough, that cuts the RF connection.
And tests in the “wild” are also usually required. I guess, in the past 12 months, I had several “good versions” but the wild demonstrated the opposite ;-).