You would need to get a problem client MAC address and review the logs on your AP as well as a Support Info when they say they're having problems (if it is a Wi-Fi problem). Check that client's connection data. This client for example has low RSSI (17/12) which means they are far from the AP. But, they are reporting 77 PER (packet error rate) which means they are dropping 77% of the packets seen. PER is due to interference near the client.
If there is a large area being covered by this one AP, a second AP might help offload usage and improve reception for users. You could configure an identical WLAN on the second standalone AP (of any model).
But, if Wi-Fi is not the issue... How is the general Internet connection for the wired network clients on the same switch as the AP? How big is the WAN link, and how heavily utilized? If this Sage app talks to an Internet server, the LAN/WAN could be the true underlying problem.