Dell G15 5515, unable to get ideal SSD speed (Samsung 970 EVO plus 500GB)

You mean to say that I have to use a M.2 2230/2280 solid-state drive? But Samsung 970 EVO is already a M.2 2280 SSD. I've seen on Youtube people installing 970 EVO plus in their Dell G15 (but no one has shown a speed test on their laptops)
I didn't understand what you mean exactly.

Your must replace the drive with the same form factor as the original.  If you did not order the second SSD drive, the slot may not work properly.  Caveat for your system storage specs can be view here.

I read this part, what I understand is that if my laptop came with only 1 ssd installed, I cannot install second one.(correct me if I am wrong). But the Dell website says that the Laptop can be upgraded upto 2Tb. 

Also the laptop detects the second SSD so the point in which it is said that we cannot install second SSD is discarded. I still dont understand what the problem is exactly? I suppose this is a software issue? Please help!!!

I think I got your point, that the laptop's second slot might not work properly because It did not came with 2 SSDs installed. Is this what you are trying to say?

So the issue is resolved. I think it took some time for the laptop to configure the BIOS before unlocking the full potential.
My laptop got updated 2 times today and after that the speed test works as expected.