@girish & @mehdi - thanks for your advice! It gave me the idea to Tailscale. I installed it on the Cloudron server and was able to successfully mount a shared folder from my Synology using cifs. However, even though the data persists between reboots, I don't see the data on the Synology, even when I'm logged in as the root user. I'm not quite the linux expert, but I noticed that when I changed Nextcloud's appdata folder to point to the mount, it created it as a "root" user. On the Synology, I created a Cloudron user that has the necessary permissions and I mounted the shared folder with the Cloudron user, password, and domain. So, I'm thinking this has something to do with user IDs not matching up or something like that.
I'm nervous about moving forward since I can't actually see the data on my Synology. Would you or anyone else have any advice?
By the way, this is how I mounted it:
sudo mount -t cifs -o credentials=/etc/nas-credentials,vers=3.0 //nas/Cloudron /mnt/nas
In fstab, the command is:
//nas/Cloudron cifs -o credentials=/etc/nas-credentials,vers=3.0,_netdev,auto 0 0
Thanks for all your help!