Yeah that sort of accuracy for civilian use can only be achieved with a near by base station. I'm lucky, there is a free base station service about 15km away. Anything under 40km away is good enough.
If I was any further away from a free base station, I would need to set up my own base station, which isnt hard to do, its the same board, but configured differently. So I would need 2 SimpleRTK2B boards instead of the one.
The disadvantage of using the government free base station is the rover needs to be connected to the internet to get the RTK correction data.