Poor mobile internet coverage in the countryside (in the forest) how to strength

Hello,
I know that there were similar topics, but I could not find a clear answer to my problem. In addition, I am a layman (and a woman: p).

The problem is as follows: I have a plot of land in the countryside (60 km from Warsaw) in a heavily forested area. I took my laptop there with the thought that I would work on it there. However, the coverage of the 3G signal is too small. The nearest BTS (T-mobile) transmitter is approx. 6 km from my plot, another (Plus) approx. 7 km, the other way. I tested the cards of these two networks and Orange, the latter, surprisingly, the best and is 10 km away (data according to http://beta.btsearch.pl/).
The laptop is an old ThinkPad with a built-in modem (the modem itself usually catches the range better than a phone in which I can forget about 3G on the plot at all and talking is poor).
The "best" coverage is in the attic (first floor), only in some places, not much on the porch (outside) on the other side of the cottage.

I am asking for help, what antenna should I buy to improve the range? There may be a set: antenna, cable, (external modem?), I do not have to move with it.

Welcome
An additional external directional antenna can help as much as possible, it is difficult to say which will work best for your location. It depends on the frequency at which the modem will communicate with the BTS station, but it is assumed that the most popular 900Mhz works best in difficult terrain.
And here you can actually choose from two antennas, the smaller and weaker ATK10
and the bigger one, stronger (attention! 1.7 m long) ATK20
You can easily buy them by mail order, and stores often offer them together with
with the installed cable terminated with a suitable socket. I don't know what modem you have, but it must have a socket for an additional antenna. For this socket you need the appropriate "cable" connector to connect the external antenna.
The external antenna must be mounted as high as possible and aim it at the selected BTS station.
And something else, you can also buy a WIFI router with a GSM modem socket, such a set makes it easier to use the Internet, we do not have to "drag" the antenna cable with us.
And an example of the antenna shop I mentioned:
http://www.cybertech.com.pl/k8,anteny-lte-umts-gsm-anteny-hspa-2g-gsm-900mhz.html
connectors:
http://www.cybertech.com.pl/k9,konekty-i-kable-antenowe.html

Of course, the 900Mhz antenna is one example of possible solutions, it happens that people even use LTE technology despite the fact that BTS is covered by trees or forest, the rule is not here.
greetings

You need to obtain the CID and LAC number and it will be known (if the base in that area has been updated) which transmitter you are using. Orange and T-Mobile were modernizing under the joint name of NetWorkS! and theoretically the range should be similar, because Orange was launched from the T-Mobile station, and T-Mobile from the Orange station.
I don't know what the modem management manager looks like in your hardware. I always use USB modems and the following applications:

http://www.nerve.org.za/mdma/

http://wiki.bez-kabli.pl/index.php?title=Top_NetInfo_-_FAQ

On the basis of the above data and the received signal (expressed in - dBm) it can be determined whether the antenna should be at 900 or 2100 MHz.

Thanks for the answers! In case of what, I will ask. Greetings!