My recent contracted 100 Mbits DSL service is slow! (5~6 Mbits)

Hi,

I've tried to contact the hotline english support but they said that they don't have people to support DSL services in english.

The point is that my 100 Mbits DSL service is very slow.
I've tested many times and all the tests results are getting 5~6 Mbits (using the official internet test recommended by o2: http://breitbandmessung.de/test with a ethernet cable).
The connection speed guaranteed by contract it's something between 50 and 100 Mbits.

Could someone help me please?