iMacs lose connection to windows server

We have recently moved over to macs in the office, so previously we had 3 PC's all connected to a WD Sentinel server where we access large files between us rather than keeping information on the computer.

We have slowly been moving over to iMacs but the problem is these lose connection to the server, so for example if we are transferring large image files, they will just stop, or if accessing catelogs on Lightroom that are stored on the server, we have to 'find missing folders'. This can happen every hour. The connection is also overall slower (for example loading previews/thumbnails) compared to the PC's, which arent even spec'd as high as the iMacs. It also has nothing to do with it going to sleep, it can happen in the middle of working.

All iMacs are connected via ethernet as the PC's were. The WD is meant to be compatible with macs and pc's, which is why we bought it.

Ideally I want to avoid having to splash out money on a Time Capsule and connect them all via thunderbolt, but wanted to check if this is my only option?