This almost always comes down to local network issues outside of dCS’s control. The most common reason for this is that the Internet Service Provider router most people use blocks the traffic that a dCS unit uses to communicate with the control device running Mosaic. Unfortunately, as these routers are mass-market consumer products, there is little scope to change how they behave in this regard.
Thankfully, simple unmanaged network switches have no issue with this traffic and can be a great way to sidestep the issue. There are plenty of very low-cost options available from reputable network equipment manufacturers.
Alternatively, you may be able to replace the ISP router with something more capable. While the ISP router usually does just fine (particularly given that it is a free bit of kit!), changing to a third-party router will often give a more stable network and avoid device discovery issues with Mosaic.