That is not looking good. With that low SNR margin, you might even have an unstable 6Mbps connection with that Netgear modem.
Usually I suggest that people downgrade their line speed till their SNR margins are 10dB or higher.
Technically an ADSL connection can only be made when the SNR margin is 6dB or better, so perhaps the Noise margin in the Netgear modem's case is SNR margin - 6dB? I think I've seen people saying that this is the difference between SNR margins and Noise margins...
Even if you switch to G.DMT, I highly doubt that you'll ever get 8Mbps.
If you can somehow find out at Telkom how far you are from the DSLAM, then you can estimate what your downstream line attenuation value should be, where it can be calculate to roughly 14dB/km
So if you're like 1km away from the DSLAM, then you definitely have a line fault. If they tell you anything between 2 to 2.5km, then your line seems to be OK to me.
Also, have you ever been connected at speeds higher than 4Mbps with that very same ADSL line? Like if you know you've been connected at 10Mbps speeds before, then you should let Telkom fix it, in which case you'll notice a drop in your line attenuation values.