I have installed my portal 10.4 on a server that doesn't have access to the internet.
So definitely my portal cannot access the online basemaps.
Can anyone please share the exact steps on how to add an offline basemap to a webmap in portal 10.4?
1. you have to publish your own basemap through your ArcGIS Server, the service type is map cached. you can follow to this url for the tutorial : Tutorial: Creating a cached map service—Documentation | ArcGIS for Server
2. to change your portal basemap gallery use your local basemap, you can follow this documentation : http://server.arcgis.com/en/portal/latest/administer/windows/customize-basemaps.htm