Calibre (not web) connection to device

I have linuxserver/calibre running via Docker on my Synology. Works great.
However, when connecting my e-reader KOBO Glo (yes, an old one) there does not appear a “sent to” icon. So I am unable to export my books to the KOBO. I activated the Kobo plug ins.
Is this function not included or do I have to add something else?