If you are running wired gigabit on the OBS side, that is good. As for the WiFi, you want the best that can be handled between the phone and router. Have as few other devices using WiFi as possible and make sure you are using wireless spectrum that isn't being used by another access point nearby, as that can cause interruptions.

You don't mention how much delay you are seeing, but with NDI-HX there will always be a little bit which is unavoidable because of the Long GOP compression. Test using NDI Studio Monitor (just to make sure OBS isn't adding additional latency). I did a quick test on my iPhone into NDI Studio Monitor and I'm seeing around 250 milliseconds of delay. (About a quarter second). I'm not sure what is the theoretical lowest you can get, but I'm guessing probably not that much lower than a quarter second.