From what I understand if you see pixelation or blocks or squares on the internet channel that's because the data stream sent for the picture is picking up errors along the way which can't be corrected by the box.
So, why are you picking up those errors somewhere between the telephone exchange and your TV box?
Probably lots of possible reasons, but in my experience I've seen two cases.
Firstly I've seen very regular picture breakup, once every two minutes consistently. For me, swtitching the powerline adapters off and on made it go away for a good few months. Not sure if you're using powerline adapters.
Secondly I've see the central heating boiler kicking it cause picutre breakup. The boiler sends a burst of radio interference which disrupts the data stream to the box. Could your picture breakup be down to something similar, maybe some electrical applicance or light being switched on or off? If so, fixing the appliance is the best you can hope for.
None of this may apply to you or maybe I've struck lucky and something will, but good luck in sorting it out!