I’d like to mention, should it matter, that as of firmware v 4.3.7-release4 for the Certa (GL-AR750), NAS/SMB/MiniDLNA was dropped. v 3.1.6 should still have it.
Otherwise one would have to get into LuCI & SSH to manually set things up… but like most things OpenWrt, it can usually be done. Here’s a far more advanced setup demonstrating how to expand the OS fr the quite limited onboard 16 MB flash to, well, whatever size you want. I set 1024 MB (1 GB) & even this seems overkill: