Have you switch off the button of wifi of your PC? or do you have installed the driver? you should get into management of your computer and update your network adapter.

where do you get it only connect 54 Mbps? TL-WN881ND really can be reach 300Mbps if the device have Successful negotiated.can you set dlink router to 11n mode.and maybe they can negotiate the speed...

less electromagnetic interference,less hamper and it's better if it can be view directly from router to adapter.

the range of the TP-WN781ND can be reach 20 meters.but make sure there are less electromagnetic interference and less stumbling block. It's better to your network if it can be viewed between router...

just disable DHCP which you can set in configure page and be aware,don't use the wan port whose color is blue.