Re: why is the installed SD not detected
The phone can't read an NTFS filesystem nativelyk, it needs an app to do that. If the card is 32GB or smaller, format it as FAT32 (which it should be already). If it's 64GB or larger, format it as exFAT (and forgive my jealousy).
I suspect, however that either the card is defective or the card slot is. If the card works in a computer, it should be recognized by the phone, even if it can't be read. (If you got the card at what seems a very low price for the size and speed of the card, suspect that it's a defective, cleaned and reprinted dud. You can buy "factory refurbished" [not according to the factory, though] 64GB SanDisk Ultra cards for as little as $10 from some vendors. And they really do look like SanDisk cards - until you try to use them. Even the packaging looks authentic.)