Only your map project is exported - no basemap of any sort. If you want a basemap on your Lowrance device, then probably the best way to achieve this is to include an OSM basemap. For this, you will need to download an OSM data file for your area of interest, and include it as part of your map project export (there is an option there, and you will need to locate your OSM .pbf file during export). Note that this will slow down your project build considerably, so another option is to create a basemap AT5 once only, and then keep this at5 file on your SD card alongside your exported RM maps. They will both be shown together on the unit.
If you search this forum you will find threads with more detail on how to find OSM data files and build them using either RM or the IMC directly.