Can't connect OP-1 to USB OTG

I would like to manage OP-1’s files using an Android phone - rename, backup, upload tracks and so on. I tried to connect it to Nokia 7 Plus and Asus Zenfone. They support USB OTG and have latest Android version. Unfortunately both say that “Teenage” disk has incompatible filesystem and must be formatted first (which I didn’t proceed aware of potential problems).

My OP-1 is new, purchased just two weeks ago. The firmware is upgraded to #237. I use the original USB wire. When connected to a PC in DISK mode my OP-1 is discovered and I can manage the files.

My phones smoothly connect to various pendrives - both FAT and FAT32 filesystems. No problems about it.

Why the phones cannot read OP-1 disk? Could anybody advise any solution?