Hello,
Add both raster to ArcMap
Open Windows -> Image Analysis
Select rasters in Image Analysis window (not in Toc)
Use key in Processing window Composit Bands
You will get the virtual combined raster which you can export if necessary
Should be like this:
Create vector layer with tower locations with heights as Z values.
Convert vector layer to raster with same extent as DEM raster.
Add tower raster to DEM raster in Raster Calculator (same name in ArcGIS and QGIS).