Does it possible work on OpenEMR work offline and sync when online
Welcome @1ntizor !!
This has been done by many people in the community. It depends on what is available to you for backup options. Which hosting platform are you using will determine what online backup you are using.
In one of our setups, we used AWS and Google Cloud. We have AWS as the primary server. At 1 AM the database is dumped from the AWS server. We use rsync to transmit the file to the GC server and import the data there. In the morning the two are identical. This is just on possible way to do what you asked.
You can dump the database on your cloud production server and rsync it to a local machine. Then store that local file on a sinology.
The sky is the limit.