Wifi early 2009 iMac

Ver 10.11.3 2.66GHz 8GB RAM

Problem is computer shows it is connect with wifi to router, getting an IP address, subnet mask and router IP. When trying to ping out with wifi, get 90% packet loss. Hook up Ethernet cable and turn off wifi no packet loss through router. New Readynet WRT500 router 2.4 only. Cx has other devices, iPad and iPhones, Apple TV, they do not have any problems with wifi connection. Trying to figure out how I can trouble shoot wifi problem in iMac.

Thank, Doug