Question not clear. But to insert Latex Matrix (or any other latex output, such as equations etc..) in textcell using the notebook, here is an example. If OP does not want to use the notebook at all, then this is not possible and will delete this.
Load Matex to render the Latex. Start text cell. Insert the latex in the textcell from the rendered output. Here is an example

Code:
<<MaTeX`
matrix = Table[i + j, {i, 1, 3}, {j, 1, 4}];
matrixAsLatex = MaTeX@TeXForm[matrix]
However, I would not do it this way if I were you. I use Mathematica all the time to generate Latex, but I output direct Latex to a file which later compile using lulatex directly. I use this method to generate hundreds of PDF files and reports all from Mathematica. (I do not use Export Latex at all either). But to do this, you need to know little Latex and make heavy use of TeXFrom as well when needed. Will try to post an example of how this method works if needed, but you asked for using textcell.
An option between using texcell and full method I use is to render the whole thing as String in the notebook. Here is an exammple
ClearAll[y,x];
<<MaTeX`
matrix=Table[i+j,{i,1,3},{j,1,4}];
matrixAsLatex=MaTeX@TeXForm[matrix];
equation = y'[x]==x;
sol=y[x]/.DSolve[equation,y[x],x];
ANd now type
string="\\text{My Matrix in Latex is} "<>ToString@TeXForm[matrix]<>
" \\text{which looks nice and my equation is } \\fbox{"
<>ToString@TeXForm[equation]<>"}\\text{ which Mathematica can
solve giving the answer}"<>ToString@TeXForm@sol;
MaTeX[string,Magnification->1.3]
Which gives

Or you can use Row
instead, as in
string=Row[{"My Matrix in Latex is ",
MaTeX@TeXForm[matrix],
"which looks nice and my equation is ",
MaTeX@TeXForm[equation],
" which Mathematica can solve giving the answer "
,MaTeX@TeXForm@sol}]

The method I use, does not do the above, but sends all output to file. So it is basically as if one is writing plain Latex, but sending output to file, and using Mathematica to do the computation and convert results to Latex as needed.
<< MaTeX
` thenMaTeX@HoldForm[Sum[1/k^3, {k, 1, \[Infinity]}]]
$\endgroup$