Split a Cell Diagonally

Have You read the following article? This is the only answer i can find on this topic.

https://support.office.com/en-us/article/merge-and-unmerge-cells-5cbd15d5-9375-4540-907f-c673a93fcedf?redirectSourcePath=%252fen-us%252farticle%252fmerge-cells-c564ae6c-3d9e-4512-8d24-1ad3b21f089a&omkt=en-001&ui=en-US&rs=en-001&ad=US