How do I combine two text values from different cells to equal another cell?
I would approach this a little differently to (hopefully) simplify things a little!

Let's assume you will use Rows 4, 7, 10, etc to enter the text of your words/sentences (which is the approach in your screenshot).

Let's also assume that you're only going to use Columns C to W to enter your text into (although there's no reason why it can't be more columns, but this is just what I gather from your screenshot).

For this answer, let's assume your Alphabet is in Row 50 and your Substituted letters are in Row 51. 

In summary, this means:


the formula you need will have to be entered in every cell from C5 to W5, C8 to W8, C11 to W11, and so on
your alphabet will be in Row 50
your substituted letters will be in Row 51.


So, let's build this using the following steps:


In Row 50 add the letters of the alphabet from A50 to Z50
In Row 51 add your substituted letters
Enter the following formula in cell of C5:


=IFERROR(LOOKUP(C4, $A$50:$Z$50,$A$51:$Z$51),"")

(The reason we're using IFERROR in the formula is so Numbers will ignore any spaces in your sentences, rather than displaying an ugly error icon.)


Copy and paste the formula from cell C5 to your other cells (i.e. cells D5:W5, C8:W8, C11:W11, and so on)
Now start entering your text to be ciphered into cells C4:W4, C7:W7, C10:W10 and so on
As you enter the text, the ciphered result should appear in the cell below. If it's a space, the cell below will remain blank


NOTES:


Be careful to only include the letter and nothing else in each cell of your Alphabet row. Having a space or anything else will cause problems. For example, if you entered your E with a space after it, then Numbers will never find the E because it's looking for an 'E' and not a 'E ', if you get what I mean.
If you have the need to have no substitute value for some letters of the alphabet, then the result will be a 0 (i.e. a zero is displayed) instead of a blank cell. If this applies to you, you could create a conditional highlight to hide the result.
Personally, I would actually put your alphabet and substituted values in another worksheet, but this may not be necessary for your purposes.