router disconnects repeatedly

For 2 months my home router (homebox 6441) repeatedly disconnects or connects with no internet. It has been fine for years, this is a recent development

The O2 My Service app keeps telling there is no problem when clearly there is. It does say however that the bandwidth is low, and gave me a vorgangsnummer (758456) Right now that app won't accept my router serial number password even though it accepted it yesterday. There is no English-speaking technical support, the lady I spoke to told to get someone else to call. So I am hoping someone at O2 sees this and helps. I cannot work from home anymore and have had to rent a hot desk in a shared space. 

Edit by o2_Katja: Moved to our English Community