D-Link DAP-2553 User Manual page 32

Section 3 - Configuration
HT20/40
Enable this option to reduce interference from other wireless networks in your area. If the channel width is operating at 40MHz
Coexistence :
and there is another wireless network's channel over-lapping and causing interference, the Access Point will automatically change
to 20MHz.
Transfer DHCP
Enable to transfer the DHCP Offer to Unicast from LAN to WLAN, suggest to enable this function if stations number is larger than
Offer to Unicast :
30.
D-Link DAP-2553 User Manual 32