The Netgear DGN2200 modem is terrible for 4Mbps connections with such high line attenuations.
I have no idea how good the Billion modem is, but my guess is that it is just as bad.
See if you can borrow someone's D-Link or TP-Link modem instead. If you'd want your line to be stable, then you'll have to phone Telkom to lock your profile on a certain (lowish) speed, eg. 3Mbps
If you keep it at 6Mbps, then the slight fluctuations in the already low SNR margin would cause the modem to resync, which is what you're experiencing now.
ADSL can only synchronize if the SNR margin is 6dB or higher, so I usually advise people to ensure that they should try to keep theirs above 12dB.
For each 1Mbps that you decrease the downstream speed, your downstream SNR margin should increase with about 3dB.
In the rainy season, ADSL problems occur more often, especially when some of the manholes gets flooded, and then even a good modem won't fix the issue.