First of all specify what it is you intend to do with your system.
Try to calculate how much data you need to transfer per calendar month.
After calculating how much data you are likely to want to transfer you can decide
which cellular services your system is going to require.
Choose the most appropriate method of telemetry for your project a GSM modem might
be unnecessary and inappropriate.
See Choosing A Data Transfer Technology
Is the installation going to be inside or outside.