Slow DSL Connection

Hi,

I have a 16 Mbps DSL contract with O2 since about 4 years ago. I was only able to get 12 Mbps in the past, but fine. The problem now is that, since a few days ago, I only get 3 or 4 Mbps. I've tried resetting the router numerous time, jiggling the DSL cable, and even did a factory reset of the router yesterday - still, I'm stuck at 3-4 Mbps.

I've just tried to call the hotline at 089787979400, but the guy that answered was unable to speak English to me and also refused to transfer the call to an English speaking colleague (I was under the impression you guys offered English support on weekdays, between 11 and 15 - has that changed?). So now I'm not sure what to do. This is a tricky issue to explain with my limited German skills, so I need to speak with someone who understands English. Any tips or advice?

Thanks.