Hello @AndrewM and welcome to the forum!
I don’t have FTP set up , so can’t give details on how FTP would fail (there are also two versions available) but the general plan is that it tries as scheduled, gets what it gets, and lack of connection will result in error.
Backup only when host is reachable; retry backup that was missed due to connection issues has discussion and ways one can use Scripting options to avoid attempting backups when the environment appears wrong.
If you are security conscious and the NAS allows, you might consider SFTP. FTP is usually unencrypted and does not validate the identity of the server, so a risk of exposing FTP’s credentials or transferred data exists. Exposure risk can be relieved, respectively, by not reusing passwords, and by having Duplicati encrypt data.
If you wish to ensure you keep backups at somewhat regular intervals (regardless of connectivity) you could schedule them more frequently than needed, then set up backup retention rules to thin them out afterwards.