I have very similar results from this account. I have the 6mg uncapped account. To put it bluntly, its crap.
I think its free because nobody would pay for this crap. None of the cell phone games work at all(ie COC and so on), games like POE are simply unplayable due to such a bad ping, whatsapp and other messaging apps dont work, dont even think about netflix.
Now I see they are advertising that they are changing all their accounts to "uncapped". Bah. What a load of BS.
I managed to get my line working perfectly. I called Telkom to recreate the connection on their side. I purchased an unbranded modem, previously i used the Mweb modem. Lastly i complained to FNB connect. They were extremely helpful and tracked the line for a few days until it worked perfectly. Touch wood its working without issue for the last two months.