Omni symbols

In addition to 2G/3G/4G, you can also "NB" are above the field strength (signal strength). What does this mean?

NB stands for NB-IoT and means "Narrow Band Internet of Things". 

NB-IoT is used in the areas of smart metering, asset tracking, environmental monitoring, smart cities and industrial IoT, where low power consumption and reliable connectivity are crucial. 

At Stromer, it is a fallback option in the event of a missing mobile phone connection, which Stromer offers with the FW-SUI 4.4.3.3 (20-11-2023) and presumably serves to protect against theft. I am not aware of any further details.