There’s no easy way to tell your Mayfly sketch to wait longer on initial startup, the instructions for that are buried deep in one of the supporting library files and not in the main sketch. Usually just having the larger antenna and using it in an area with known strong cell signals during the initial activation is all that’s necessary.
One other thing to check is that you’ve inserted the SIM card properly in the cell module. The first photo in section 6.1 of the manual shows the right way to insert the card. There’s a little notch in one corner of the card, and that’s the end you insert first, and with the gold contacts of the card facing the green circuit board and the text on the card facing away from the circuit board, as shown in the photo. Sometimes people accidentally insert the card upside down or backwards (or both upside down AND backwards), which obviously will prevent the board from making a connection. We’ve also seen antennas not properly snapped all the way onto the tiny little antenna jack, resulting in connection issues. So just make sure everything looks the photos shown in the manual.