This is the solution that I have found for my Marantz AV8802 periodically not showing up in the "Connect to a Device" Devices list.
Setup Menu of Marantz;
Setup-Network Settings-Set DHCP to OFF.
Cycle power to device for 1 minute.
Let this run for a couple of minutes, it doesent always complete but just select back to get back to Network menu.
Setup-Network Settings- Turn DHCP to ON.
Setup-Network Connection- Wi Fi setup. Select correct Network, set PW.
Check all Spotify devices and they all should now see the Marantz again.
This seems to last for a few months and then for some reason Spotify will once again not see the Marantz and you will have to repeat this process.
Prior to this I inusred all devices were at latest update revisions and had tried un-installing and re-installing Spotify connect on all devices.
This was the only solution that works for me.