Fill a hole between outside meter box and wall

Buy insulation spray that will do the job leave it dry out and expand then cut around edges with Stanley blade

All depends what the house is built with , if built with brick or block then could cut out to stitch other block or brick in to replace and brick up to match exact size of meter box . Or could fill up with cement if realy needed too so can stop dampness getting in.could also use expanding foam and cut back tidy when dry. Many ways of closing the gap

Expanding insulation foam (blue ) let it expand cut it flush then get upvc trim (whatever colour you need) stick it on and around gap then cap round it Tip Dont go over board with foam it will expand so go lightly

Ideally would need toothing out and filling back in with bricks/blocks as per original building

Insulation spray

It sounds like you have a gap between the meter box and the brick wall that needs to be filled to prevent water from getting inside. If the gap is only about an inch, then you could use brick caulk to fill it. Before applying the caulk, you should clean the gap thoroughly to remove any loose debris, dirt, or old sealant. You can use a stiff brush and a vacuum to clean the area. Once it is clean and dry, apply the brick caulk according to the manufacturer's instructions. After filling the gap with the brick caulk, make sure to smooth it out with a caulk smoothing tool or your finger. This will help ensure a neat and tidy finish. It is also a good idea to check the sealant regularly to make sure it remains intact and undamaged. Regarding the dampness on the inside wall, it is possible that water has seeped through the gap and is causing the dampness