I am not exactly sure what you are trying to say but I think you are complaining about the internet speed. If you are complaining about the internet speed at home then the second part should read '... speed of 5GB as well as two mobile phones ...'. If the internet is on the mobile phones then you should say 'I signed a contract with you for 2 mobile phones with an internet connection of 5GB'
'However there are often interruptions in the internet service.'
Hope this helps