I don't know the general help number for G4 internet problems but you could try using
this page to find your nearest store and then phone them. If you put your post code into the box, remembering to separate the first 3 and last 2 number with a space it should give you some options near you. Select one and you should get a map with red markers on it. Hover over any marker and you get a box with a control labelled "Περισότερα". Click on that and it will pull up the details of that store including a landline number.
I don't use G4 internet but doesn't it have a monthly cap and if you exceed the cap doesn't it drop you down to a slow speed rather than cut you off? Don't you have an online account/app that you can log onto and see your usage to check if you have gone over the limit?
Warwick
PS Perhaps your nearest mast is self isolating?