Modem XB8 can't provide 2 GB internet speed

We upgraded our internet plan to 2GB and got the newest modem XB8, and so far, we could not get what we paid for (we tested the internet speed, only can get 900+ Mbs). The Xfinity technician came today, he test the coming in signal (directly connect the coax cable to his device), and get 2500 Mbs, but once he connected his testing equipment with his ethernet cable (he said it is CAT 5e) or our cable ( CAT 6), he can only get no more than 1 GB speed. Also, he replaced our modem with one he brought in, same result.

So can anyone solve our problem?