Same here. Need it for work, can reach their main website, but can't get a connection to our Slack workspace - also having issues connecting to Dropbox.
Using OpenServe fiber.
I changed to Google DNS 8.8.8.8 / 8.8.4.4 in my router setup and that has worked around the issue for me. There is something odd with the Cool Ideas DNS servers in Cape Town - in my opinion.