Countries frequency bands

[wibble]

But let me understand one thing, maybe it is possible to change the imei as you do for the mac address of the network cards?

where is this information encoded and can't you change software like you do for macs inside network packets?

It's meant to be a fixed, unique identifier for the device. In some jurisdictions it's illegal to change it without authorisation, in part because most of the reasons to change it involve theft or fraud. In this case I'd expect it to be baked into the modem module, and that the manufacturer won't make public any tool to change it.