Me on my Windows 10 pc and my girlfriend on her switch found that we are unable to join eachothers worlds through cross play when she visited and connected to my wifi. As we had played before when she was at home I guessed the issue might be that she was on the same network as me, and I used my mobile hotspot to confirm this. It worked fine when she connected to the other wifi network to play with me.

In short you cannot connect to someone on the same wifi network as you.