I have never tried to use MyTvOnline for that but the Z boxes will happily allow you to simply browse resources on your LAN using DLNA, SMB/CIFS, NFS etc.
If you are able to build a .m3u playlist for the media on your NAS, then you could possibly coax MyTvOnline to co-operate.
Alternatively, publishing a local Jellyfin server either on the NAS or on a device on your LAN that points to the media on your NAS is a very good option, and you can get the Jellyfin app for the Z box from F-Droid. (running Plex locally works in a similar way, but is dependent upon an account with Plex and everything ends up being dependent upon their web-based API's, so not totally 'local').