I've been experiencing this in every motorsport since forza 5: after a while, I just can't never find any lobby of any type of race in multiplayer, I click in a race, it searches for some seconds, and it says disconnect from servers. My NAT type is open, I only experience this in this game ( and forza 6 and 5 back in the times). I can only play multiplayer when I'm with somebody in my party/squad, then it works perfectly, and with no lagging or anything like it..