Unranked lobbys are fine for us, we don't have connection problems in there. That is the solution we use now for our league races.
But because of the league mode comes with extra features, like saving and all the other stuff we prefer the league mode.
It doesn't matter who creates the league and shares the code we can not join it sadly. A public league is not possible because we are unable to drive in the F1 2020 cars.
I can create a private league and share the code in a PM if that helps to replicate the issue?