How to import photos from a digital camera using USB OTG?
The camera may be connecting on PTP (Picture Transfer Protocol) or MTP (Media Transfer Protocol) mode, not as a normal USB drive (which uses USB Mass Storage drivers). It shows up in Ubuntu similarly to a USB Mass Storage device, but Android has no support for this out of the box.

You could try an app that supports PTP, such as DslrDashboard, but I haven't tried these personally, so no guarantees.