Perhaps, bottlenecks at the WAN interface to ISP should be monitored, but backup here should probably affect all application users.
When wired performs well but not wireless, you need to look at the particular AP that clients seem to be having problems on. The AP support info file, has an Athstats section for 2.4g and 5g radios that will show if there is an interfernce source near the AP, when changing channels might help. They also have individual connected client MAC details about RSSI and PER that are user location specific, related to distance from the AP and possible interference sources near the client. Do all clients on that AP have a Skype performance problem at the same time, or do only 1-2 of the total have a problem? This is what to investigate to get an idea if client wireless connections are impacting the application, and if it might be AP or specific client related. You need to collect the specific AP's support info, when users are connected and reporting problems.
Hello! You should see the amount of users connected to the AP and if you can do it, you should do a test with ping to check the times, the voice is very sensitive so if your times are kinda high the communication would be really bad.