Sorry, I just wanted to explain my answer. Didn’t meant to confuse you.
If you have your data on a Network Storage, you can create a vault on the same network storage with the Cryptomator app. You can create a vault at any place that is available for your OS and your user.
When you open your vault, there will be a new virtual drive that might look like a „local“ drive, but in fact your actual vault files are stored where you have configured the vault to be stored. In your case that’s the network drive.
Then place you unencrypted network files into the virtual drive to encrypt them.
BUT: you mentioned a poor network connectivity.
When you place an unencrypted file (from your network storage) into the virtual drive for encryption, the following will happen:
The file content will be loaded via your poor connection to your local Cryptomator app. Cryptomator will encrypt it and will generate the encrypted vault files. As the vault is stored on your network storage, the encrypted file will be transferred back via your poor connection to the storage.
In short: yes it is technically possible, but with a poor network connection be prepared for a time consuming process, maybe even Timeouts. You can avoid Timeouts by increasing the dokany timeout threshold
(you should try if this is really needed. Maybe the connection is not as bad as It sound 
)
It is not possible to encrypt files with Cryptomator directly on your network storage, means you’ll always have to use your connection bandwidth.