If I undestood correctly, @karlos_devel can mount the card correctly when it is inserted to the USB socket but not when it is inserted to the SD socket.
(I assume it is put into some kind of adapter when inserted into USB socket, right?)

There is difference with USB and microSD, the adapter is not just a dummy piece with wires inside, it is a fullblown microcontroller that adapts the signals from one type of bus into another, handles timing, caching, access selection, etc...
Hence this only proves that the card works correctly.

Now, there are tons of reasons why it might not work in the microsd-slot.

• it might not connect electrically correctly (have you tried other cards? The same card with different filesystem, like exfat?)
• the filesystem might be somehow corrupted or aligned differently, in such way that the USB hostcontroller in the adapter recognizes it but the phone doesn't

Is the card recognized at all when it is in the slot? Do you get anything about mmcbl1 in dmesg or journal?