Depending on the router you use, Most of Teltonika devices can operate on 4G perfectly fine, when a single antenna is attached to the connector marked as MAIN. The other, AUX connector, is used for additional signal processing to improve the received signal. A different gain antenna might be connected, but it will probably not improve the connection quality greatly.
Regarding the length, as long as the input impedance is 50 Ω, it can be connected. In your case, if the cable length is actually 21 meters, you should consider a high quality, low loss coaxial cable, as the signal might be attenuated rather greatly over such a distance and diminish the gain provided by antenna.