1 - Make sure all static meshes on the project have nanite disabled.
You can do this going to the content folder and filtering by the asset type, then select them and disable nanite with right click → Nanite → Disable for selected. If you wish to be more careful with modifying hundreds of meshes (like, due to source control), use bulk edit via property matrix. You will be requested to apply changes for them afterwards.
2 - When disabling Nanite from the project settings, the first of the following lines is added to DefaultEngine.ini . Add the second one manually.
r.Nanite.ProjectEnabled = false
r.Nanite = 0