It should work ok with only two antennas, it is important to guess which are the exact SMA ports for working with two antennas.
When trying which SMA combinations work best, you have to reboot your router every time and see if the internet works. If the internet is working, you need to monitor it for a few hours or a whole day to see if it is stable or if there are strange outages, interruptions, etc. So that requires patience.
Make sure that the external antennas are pointed precisely towards the base station (tower).
The better the signal (RSRP, RSRQ, RSSI and SINR), the better the connection stability and speed.
If you buy two more antennas, you have to be careful how to mount them, because then you have a 4x4 MIMO configuration.
In the link below you have my 3D picture of how to mount 4 external antennas, there must also be a proper distance, for example 34 cm between each to amplify primarily the B3 band.
If none of this improves the signal, you can always sell external antennas.
How far is the base station (tower) from you and can you see it or are there any obstacles?
EDIT:
If you have more base stations (towers) in range, you can turn the current external antennas towards another base station and test the signal and speeds 