I warmly welcome.
I watched your discussion and the problem is close to me. It just so happens that I have a similar problem when it comes to choosing the antenna for the modem and if it is possible I attach it to the topic above.
For now, I am using the standard HUAWEI E173U-2 modem and my maximum internet speed (I use the NJU MOBILE internet called NJU INTERNET with LTE with a data limit of up to 40 GB / PLN 29.00 / month), download: 5.60 Mb / s and sending : 4.06 Mb / s (but my modem does not support LTE and these are not LTE speeds). So an acceptable standard.
I inquired at NJU MOBILE about the maximum speed of their LTE internet on my NJU INTERNET and was told their maximum LTE speed was 112 Mb / s (LTE) and their LTE average speed was 75 Mb / s (LTE). So far my old modem does not support LTE and I have not checked this fact yet.
On days (until the end of this week) I want to buy a modem for LTE: HUAWEI E3372h-153 from this Allegro auction with the number: 6577372391
I will buy the modem at 100%, but I am considering an antenna for it, because I just want to have a good antenna for years, if necessary, if the LTE signal is weak.
I am considering 2 purchase options. I am thinking of buying a rather disliked antenna from this Allegro auction with the number: 6024035853.
I know it has very good reviews on a foreign site: GearBest (for 18 reviews and a 4.9 / 5 rating, it's not bad in my opinion, so it's probably not that bad). You have to enter there: LF - ANT4G01 4G LTE SMA Plug 49dBi MIMO Antenna
I know that CyberTech antennas recommended by the user of mothers451 are also good.
However, it is very large, and I need an antenna whose cable can be up to 1 meter (yes, one meter). Simply, the USB modem will be plugged into the desktop computer at the front and the antenna cables will be plugged into the USB modem for 1 or 2 antenna ports, so a cable up to 50 cm would be enough if it is to be placed on the ground, and if higher, then 2 meters of cable they are enough.
CyberTech is a bit big and you also need to buy the appropriate connectors for it, because it even says that it supports modems: antenna only with an FME (f) connector without a modem connector. And this antenna is such that it can be mounted outside and it is even more advisable, and I only need an internal antenna. So I would be asking for something good, but internal.
The problem is that the Allegro antenna that I showed above has 49dBi and will it not be too much and nothing will argue if the nearest transmitter is approx. 200 meters from me? I suspect that even without an antenna, LTE internet can work well for me, because from the nearest ORANGE transmitter (the internet in "my" NJU MOBILE belongs to ORANGE) I have 0.20 km, i.e. not more than 200 meters. This is the result straight from the beta website. btsearch. pl.
Below is a featured image for btsearch insiders:

My location is: ul. Chopina 10A, 71-450 Szczecin
Therefore, as you can see, the distance to the transmitter is about 200 meters (let's say even a maximum of 250 meters).
It is the ORANGE LTE 1800 (1197) transmitter at ul. Swan as seen on the map.
And the question: Which antenna would be the best in my situation? 10dBi? 11dBi? 15dBi? 28dBi? or bigger? or maybe smaller?
Citing a short article from the site: modem antenna. pl
"Knowing the exact distances of the transmitters from our location, we can determine which antenna will be the best for our needs. Below are some examples of rules that should be followed:
- Traditional 11dBi antenna - recommended for urban areas where the distance from the transmitter does not exceed 3 km.
- Traditional 15dBi antenna - dedicated to LTE internet, where the distance from the transmitter is about 5-6km. "
My Szczecin "areas" are of course urban.
Thank you very much in advance to all "experts" (and not only) for all help and good advice in choosing the right antenna for my modem and any critical and very sincere comments about my choices. This is most welcome on your part here, as your answers may even help many future generations :)
Thank you in advance for a relatively quick response

If I forgot something, please ask and I will add the required answer.
greetings