I have exactly the same problem as the person.
It seems to be a Roon Tidal integration problem and not DNS issue as reported. The same wifi network allows me to play Tidal without issues from a computer browser but on the same wifi network Roon cannot play Tidal. If it was a DNS issue surely the problem would occur in both Roon and the computer.
This problem happens to me every few months or so and resolves itself after 2-4 weeks. During that time Tidal isn’t possible in Roon.
Can Roon tech department work on this issue and see if it can be as reliable as playback through browser?
Or at least a trick to reset any ISP locks on streaming? In the meantime I will try the cache deletion and change the router DNS (not sure if my ISP allows this or not).
Adjustment: A clue for this is that once I updated my Mac OS to Mohave, Roon starts working with Tidal again. I few minutes prior to OS update, Roon wouldn’t play Tidal.