Why no internet or slow in Dessau Center?

Why is O2 lying about mobile internet speed? I live in the center of Dessa-Roßlau and unfortunately, the internet either doesn't work at all or only reaches 100kb/s. Both my wife and I have O2 contracts. We've been living here for a month. Even in Coswig-Anhalt, the internet works perfectly, but here it doesn't. I've reported the problem at least 20 times. Even though I receive SMS messages saying it's resolved, the problem persists.