MacOS/OSX loses connection to shared NAS drive
I struggled with this for years, and eventually found out that all you need to do is turn off packet signing.
Below is a link on the apple forums place on how to do it. You need to be using SMB2/3 protocols (not AFP). 
I've never had a dropped network drive since.

https://support.apple.com/en-gb/HT205926

Simon.