TL-MR6400 V2 external antenas

TL-MR6400 V2 external antenas
TL-MR6400 V2 external antenas
2021-01-08 09:59:25
Model: TL-MR6400
Hardware Version: V2
Firmware Version: TL-MR6400(EU)_V2_181219


I have new internet provider and i bough TL-MR6400 V2,  i have problem with GSM (LTE) signal strength inside home. 

I have tried to put the router in several places in homes but max what i got is signal strength 75% and download 5-13 mb/s and upload 1-3 mb/s. 

I tried to put router outside on balcony for a while and get signal strength 100% and  download about 50-70 mb/s and upload 20-30 mb/s. 

I think about buying and external antenna but i read that this router external outside antennas (connected via SMA) is for WI-FI not GSM signal, it that true ? 

Is it any possibility to improve signal strength inside house ? (I had already tried move router close to windows and different sides of house)