What kind of external antenna are you connecting, which brand and model?
It is important whether it is a directional antenna or an omni antenna (rabbitears) and what kind of gain it has
If you have only two antennas, connect to only the first two SMA ports, leave the other two empty.
If you mix external antennas with higher gain with those omnis from Teltonika, the modem does not measure the signal correctly because one pair of antennas gives it one level of signal and the other pair of antennas another level of signal. So don’t mix different antennas.
The most important thing for you is to have antennas with the strongest possible amplification to get a strong signal, and this will lead to better speeds.
If they are directional antennas, then the distance between them is also important, I explained that in one post where a man got crazy speeds after correct antenna adjustment.