I apologize for the long response time... I've resumed this work now.
I chose to build a biquad antenna. my interest was to increase the sim card signal, so I connected only one antenna.
this modem has two antennas with two different frequencies; to find the right one I removed them one at a time, so I could see which of the two got the signal.
I desoldered the three connectors of the old antenna and soldered the cable directly onto the PCB.
I had some 75 ohm cable at home and it works quite well (the 50 ohm probably works better).
the dish is made of aluminum and for the antenna I used the wire from a coat hanger, the black support is made of plastic.
the actual gain is satisfactory, before I had 2 cleats, now I have 3-4 cleats out of 5.
I recommend this modem for those who surf with a 4G sim card; it is reliable and consumes only 2 watts (6 watts in download).
I want to thank you for the help I have received on this site, and I hope to be of help to someone with what I have posted