Ensure the default (dlink) SSID name is changed. Ensure that different SSID names are used between each WiFi 2.4/5Ghz radios. Can be anything and not something that's already in use by any neighbouring WiFi routers. Under Setup/Wireless/Manual. - I have different names
What wireless modes are you using?
2.4Ghz and 5Ghz routers: Try single mode G or N or mixed G and N on 2.4Ghz and single mode N or AC on 5Ghz? - I tried N, A and AC on the 5Ghz but best option is "all"
Channel Width set for Auto 20/40Mhz or try 20Mhz only. - channel is set to 20/40/80
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. 13 for EU regions. Try channel 48 or 149 on 5Ghz. - Strange but I can only choose channels 36,40,44 and 48. I checked 48
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 TPIK and AES. - WPA-Personal
What wireless devices do you have connected?
Ensure any devices with WiFi adapter drivers are updated.
Any 2.4Ghz or 5Ghz cordless house phones or WiFi APs near by that maybe causing interferences?
Any other WiFi routers in the area that maybe causing interferences? Link> Use a WiFi Scanner to find out. How many?