Filling external holes where cables have been removed

You can buy ready mixed in a tube from toolstation Fischer "Premium" Express Ready Mixed Cement Cartridge Sand or some brown silicone sealant you used to be able to get cement coloured sealant

Your right in thinking that sand and cement may not be the best solution. Before you do anything I would recommend using something to create and water/air tight seal In the hole. Free the holes of any debris and dust then use either a quality silicon sealant or expanding foam to create a seal inside the hole. In regards to returning the holes to a finished look, sand and cement could be used with roughly a 4 parts sand - 1 part cement mix by first wetting the area and then using your fingers with gloves to press the mix into the hole and finally touching up with a paint brush and spounge but this is prone to cracking and crumbling. Alternatively ready mix hole fillers are on the market but often times will stand out unless painted. Hope this helps