The mobile application needs access to a file directory on a server1 to interact with Emacs. Pass its location through the
org-mobile-directory variable. If you can mount that directory locally just set the variable to point to that directory:
(setq org-mobile-directory "~/orgmobile/")
Alternatively, by using TRAMP (see (tramp)TRAMP User Manual),
org-mobile-directory may point to a remote directory accessible through, for example, SSH, SCP, or DAVS:
(setq org-mobile-directory "/davs:firstname.lastname@example.org:/org/webdav/")
With a public server, consider encrypting the files. Org also requires OpenSSL installed on the local computer. To turn on encryption, set the same password in the mobile application and in Emacs. Set the password in the variable
org-mobile-use-encryption2. Note that even after the mobile application encrypts the file contents, the file name remains visible on the file systems of the local computer, the server, and the mobile device.