Do you have an Ethernet option on the Marantz? If so I would try that, even if it means running a cable for some distance from the router, just to see if that solves the problem. If it does then continue to go wired or zoom in on the WiFi as the source of the issue.
If it is the WiFi and you don’t wish to do wired, the optimize your WiFi in the home first (mesh, extenders, etc). If that is done and you still have issues the it is the Marantz WiFi receiver that is the issue