I suspect this is maybe a network issue.
A suggestion would be to take your modem and router to a friend's house with good Cell C coverage and test there.
I know it's a hassle, but that would rule out one problem.
Took the router into another area and it did not help. Now the router does not even identify the modem at all; it says "Unknown Modem Click here to configure the modem manually". I am sending the unit in for testing; I am sure it is faulty.