With all due respect, if you plugged external antenna's to the ports found outside the router, then you shouldn't be on the internal antenna. Your presumed lackluster in speed might be due to your lack of network coverage on your network operator. Might want to try and move the device close to the window or if you see a clear skied day, use it on your rooftop and see if your speeds get better.
Just as an added info, adding antenna's don't necessarily improve your speeds, if your network is crippled with too many users on it.