I suspect my Roku 3800 of kicking other devices in the house off of the wifi. This happened before with an older model and I was able to go to the wireless secret screen and turn "Interference" off. This no longer seems to be an option when I go to that menu. How can I change the setting?

Those hidden menus were never designed for customer use. Roku has always reserved the ability to alter or remove functions from them. Sometimes they get moved to the "normal" menus, but often they are removed. I don't believe this is something that has been moved to the settings menu, so it likely is no longer available.