Hi, I had the same problem with my cell phone.
I solved it in the following way:
1° mtp-detect, if the message “command not found” appears, follow these instructions:
2° https://command-not-found.com/aft-mtp-cli
3° After installing and with the phone unmounted, type again mtp-detect in the terminal.
Make sure that transfer mtp files is selected in your celullar phone.
Open the file manager and the internal storage and memory card, if any, should appear there.
I hope this will help you to solve the problem. 
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