Bad modem !?

This is my first FRITZ! Box, the more disappointed I am by the built-in modem.
Since 2 weeks I brought before my (regional) provider that the data throughput that arrives in my apartment is too small. (My tariff offers me a 50Mbit / s downloads 10Mbit / s upload, so VDSL2 with VOIP telephony.)

First it was only 28Mbit / s download with 0,7Mbit / s upload (which was ultimately a result of a lousy ables cables third-party), then was, has measured according to technicians of my telephone socket and surge protection power strip the full range on line , FRITZ! Box the connected and found that as many as 2-3 Mbit / s Upload are using the original cables at about 33Mbit / s download possible.
Well, a meter can be quite more credence than a FRITZ! Box, I thought to myself. Must be due to my cable in the house.
Everything checked what was so moved to (17-35 years old) cables in the house. Then connect the basement on the phone main connection the FRITZ! Box to check how much the laid in the home telephone cord from the power swallow.
Result: Right on the junction box, the FRITZ! Box came after all to 42Mbit / s down / 5,8Mbit / s Up. Strong improvement, but still far from 10 Mbit / s upload away!

So now I have a Zyxel VMG 1312 B30A here ordered from Amazon to the FRITZ! Box excluded as sources of error.
Now that I have tested yesterday the Zyxel router in the same way (to the telephone socket and in the basement), I conclude that the modem in the FRITZ! Box does not meet the requirements, I like that one (expensive) device location.
In the basement of the unit came on Zyxel proud 43,7Mbit / s download and 9,7Mbit / s upload. Much better!

The Fritzbox I will now keep firstonce as a router and VOIP telephony, if there occur problems, I will in the future other manufacturers AVM prefer.