The other day I noticed that my Onkyo Receiver was not showing up in Home Assistant, nor was the Onkyo Remote app on my phone able to connect to it. After some searching I came across this post suggesting a solution.
The fix is to edit the receiver’s name in its Network Settings and remove any spaces, dashes, or other special characters. You should literally only have letters and numbers, nothing else. Once I did that and power cycled it things started working again!