I'm going to assume 5ghz works , as you have not stated.
Also Assuming you can connect to the AP via a wired ethernet connection:
1) Logon to the AP using wired connection, Can you see clients connected under the Status Section under Radio 2.4G
2) Do you have the 2.4G SSID under a different access VLAN, if so check that you are being issued a DHCP address
3) Check your ethernet settinng oin the port your R500 is connected to. Can you see the wireless clients MAC's on your AP switch port table.
4) Is the right country code configured on the AP
5) Does your upstream switch have Port security enabled not allowing multple MAC addressing, and the switch has disabled the port.
These are a few suggestions. Hopefully helpful