Hello Gaessmu
Yes, exactly the additional two antennas provide better 4G reception. If the reception is ok in your area and you don't have the modem in a bunker, you don't need them. If you want to move the antenna outside, the modem also has a connection for an antenna with cable for the same function (i.e. better 4G reception). To strengthen the WLAN, you need a WLAN repeater or a mesh.

Better 4G reception ( At least that's how I understand the following sentence: "And with the two built-in high-power 4G antennas and two external antenna interfaces, signal stability and coverage area strength are greatly improved").