Mike, actually, UPnP/DNLA doesn’t operate with a permanent “connection” between the Server and Renderer (or Control point). The way it works is that the Server and the Renderer (Bartok) will discover each other by independently advertising its availability over the network via an IP multicast packet every so often (thats the “SSDP advertisement interval” configuration on the Synology Media Server - it advertises itself every 15mins by default), and then acknowledging the other.
If the nightly power shut-off fix doesn’t solve the problem, I’d suggests the same as for Erwin (though technically it shouldn’t be impacting your setup 
).
Pull down the User Manual for the TP-Link AC1750, if you don’t already have it handy.
Access the Router’s configuration Web UI, should be at its default LAN address of 192.168.0.1. Make sure to Disable both IGMP Snooping and IGMP Proxy (described at the bottom of Page 76, and on page 77).
Hopefully that addresses the issue. Can’t imagine what else could be going wrong 
One of the unfortunate problems with UPnP/DLNA; it doesn’t always work the same across various vendor implementations. 