In my Latitude E7390 I can see an empty WWAN port described **bleep** "WWAN / SSD"
Photo of the motherboard (top-left corner):
Can I place there an additional SSD harddrive like this one SATA III M.2 2242: https://www.goodram.com/en/products/s400u-sata-iii-m-2-2242-ssd/ ?
Not the one you suggested for 2 reasons. It is a SATA drive you linked to and that is a PCIe slot and that is a 2230 slot not a 2242.
From the owner's manual.
So definitelly a PCIe NVMe hard drive.
Regarding the form factor - seller of my laptop send me email that I should buy M.2 2242 form factor drive, not 2230.
OK, after further checking it appears that some retailers such as Crucial only supply SATA drives yet others such as Mrmemory state that PCIe drives are compatible. Are you confused yet...I know I am. The following posts should help. And since you have contact with the previous owner, what do they suggest as they obviously had a drive in there that was working.
And this from the tech specs
I have bought the laptop from a reseller selling regualry hundreds of refurbished dell laptops (waiting for the NBD warranty transfer to my company - then I will be able to ask Dell Support for details).
This company wrote to me that in the WWAN/SSD port I can mount PCIe NVMe M.2 2242 SSD hard drive (without any specific example model, although ie. Toshiba RC100 fits these requirements.
I have a Dell 7390 2 in1 and have installed BG2 M.2 2230 Toshiba 256 Gb PCIe in WWAN port and It works fine.
You will need a proper bracket to install this SSD as SSD is really small as compared to room available :-).
I have also upgraded my primary slot with SSD7 M.2 2280 PCIe NVMe 1TB. Both SSDs are working good.
By the way, I have made Secondary drive as my system and boot drive and have observed no difference in loading software or starting windows.