Very poor fibre speeds and interrupted feed

Our 50 Mbs line was installed on Thursday, 12 October and apart from a brief and shining moment just after installation, our line has not deliver the contracted speed. On Sunday we measured speeds as low as 2 and 3 Mbs. Beside that, our service is intermittent. It was down for hours yesterday morning. Miraculously reappeared at a speed of 6Mbs. Was monitored at about 23Mbs in the afternoon and this morning was down again. It comes and goes all the time and I am posting this on my mobile data instead of on my much promised 50Mbs line, And, yes, we logged a support request yesterday to absolutely no avail.