I'm trying to back up my user files from my Android phone (recommended before a possible phone repair) into free space on my Kubuntu 20.04.3 desktop system, but although the notifier tells me when I've connected the phone via USB cable (through a 4-port USB hub that works fine for other things including writing SD cards for my 3D printer, with a cable that the phone accepts for "charging rapidly" though I don't often try to transfer data), any attempt to access files on the phone results in an error message.
I found an answer relevant to older versions of Ubuntu recommending installation of gvfs tools, but comments indicate it's no longer needed since 13.10, as all of those tools are included in newer versions. Presuming it's relevant, I also can't download photos from my phone.
Phone is a first-gen Google Pixel, approaching four years old, and 100% functional aside from greatly reduced battery life, possible accelerometer issues (screen rotation is intermittent), and possible light level sensing issues (screen auto brightness mostly dosen't work, possibly due to a dirty Otter Box).
Do I need to do something special to transfer files (photos, music, downloaded game .apk files) from the phone to the Kubuntu system?
A setting on the phone governs how the computer interacts with it. On my Android 10 phone it's under Settings > Connected devices > USB preferences > File transfer.
Note that to see the USB preferences, it appears that you have to have a USB cable plugged in.