Thank you for the response. In that case, let me share the latest update from my research.
I installed Docker on my computer and used Docker to set up Nextcloud. I connected a flash drive to the USB port of my modem. With Nextcloud's "external driver" feature, I can access this flash drive connected to my modem. Similarly, I can also access the flash drive from my computer via FTP. Now, what I aim to do is to create a Joplin folder on the flash drive and synchronize from there. But I haven't been able to start this yet because the synchronization process on my phone is still ongoing...