Is your nas and your Mede8er connected to the same switch?
Or is there some other setup with multiple switches and routers?
Please try to describe a bit more details how things are connected and what components are used in your network as detailed as possible to get some idea what could be the root cause.
Best way to troubleshoot these kind of issues is to start with a simple as possible network setup and if that is working ok, move to more complex structure and compare again.