It's in the phone so it's internal storage. It's external in that it's not part of the original equipment but once it's in the device it becomes internal storage.I think that your SD card cannot be used as an internal storage. You may need to take your phone to someone who repairs Samsung phones and they can change the internal disk for an unknown price plus labor. You may want to check around for prices.
First step is format the SD card, should be under settings -> storage you should see the SD card and it should be intuitive from there..Follow this guide
I had my SanDisk sd card set up on my Samsung J3 as internal storage when I purchased my phone.
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