If you want it to actually be a table, so that the boxes never move onto another row on narrow screens, that’s a completely different requirement from what you said originally. In that case you’ll need to be using the HTML table tag and all that goes with that (which is a lot).
If you just need a grid, then I’ve given you what you need. You can play with the precise values in my CSS to modify it as it suits you, and you can style each box as you want (as you have).