1. If the antennas of modules are too close to each other, it will cause EMI interference which will result in diminished signal quality and raise the BER, and cause long-lasting issues with connection or connection failure. We suggest that the distance between each antenna should be more than 20cm 
2. If many modules are working simultaneously, it may prevent some modules from registering with the network due to a lack of base station resources. We recommend using the QuecScan/QuecLock function to lock different modules at different frequencies. For detail you can refer to . You can also use SIM cards from different operators if possible