The availability of Carrier Aggregation can have a big impact on the performance of your IoT SIM card.
First of all frequency bands should not to be confused with 2G/3G/4G/5G you should see them as sub options of each technology with multiple frequency bands available at any point in time.
Historically a modem would connect to a single band only which is often not a problem however there are occasions where the performance of your IoT SIM card will suffer serious deterioration due to poor band selection in a similar way to if it was restricted to a single mobile network.
LTE carrier aggregation in its most simple form will connect to multiple frequancy bands rather than just one which gives your IoT SIM a greater chance of obtaining the required bandwidth to connect to your application.
For higher throughput level applications we recommend using modem categories which support carrier aggregation to ensure you get the most from your IoT SIM card.