I would try the following connection platform first to help eliminate any connections issues between the modem and router as this is the preferred method for connecting routers and modems and other devices:
ISP Modem>DIR-857>Then network switch><all wired devices to the switch.
Some things to try: - Log into the routers web page at 192.168.0.1.
Setup DHCP reserved IP addresses for all devices ON the router. Setup/Networking
Ensure devices are set to auto obtain an IP address.
Enable uPnP and Multi-cast Streaming under Advanced/Networking.
Link>
Wireless Installation ConsiderationsWhat wireless modes are you using? Under Setup/Wireless/Manual.
Try single mode G or mixed G and N on 2.4Ghz and single mode N on 5Ghz?
Channel Width set for Auto 20/40Mhz or try 20Mhz only.
Try setting a manual channel to a open or unused channel. 1, 6 or 11. 11 for single mode N if the channel is clear.
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.
Any other WiFi routers in the area? Link> Use
InSSIDer to find out. How many?
Turn off WISH, and WPS under Advanced.
Try turning off Short GI, WLAN Partition,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.