What wireless modes are you using on the Host router and DAP? Under Setup/Wireless/Manual.
Try single mode G or mixed G and N?
Try single mode G or mixed G and N on 2.4Ghz on the Host router and single mode N on 5Ghz on DAP if your using it as an AP?
What security mode are you using? Preferred security is WPA-Personal. WPA2/AES only. Some WiFi adapters don't support AES, so you might want to try TPIK only or Auto.
What wireless devices do you have connected?
Any other WiFi routers in the area? Use
InSSIDer to find out.
Turn off Short GI and Extra Wireless Protection if you have it. Under Advanced/Advanced Wireless.
Turn off all anti virus and firewall programs on PC while testing. 3rd party firewalls are not generally needed when using routers as they are effective on blocking malicious inbound traffic.
Turn off all devices accept for one wired PC while testing.
Check cable between Modem and Router, swap out to be sure. Cat6 is recommended.