I think I solved the problem. I disabled “Enable Opportunistic Locking” in SMB Advanced Options on my Synology NAS.
@Marius_Neagu
thanks for your Info! Disable “Opportunistic Locking” solved also my Problem with permanent connection loses! 
I’m having issues with my iPad dropping the connection to my iMac running Roon. This has become a problem since I switched from my multiple AirPort Extreme home network to a new Linksys Velop mesh WiFi system. I get great coverage and download speeds but now my iPad is frequently dropping the connection. All devices have the latest software. Any suggestions?