I have a new D-Link which I was told would solve all my slow internet

"What is your internet connection? What did you order with your ISP. The data rate you bought is the limit of your connection.

Most routers offer an Ethernet connection which you can use a Cat 5+ cable to connect to your Mac mini instead of using the WiFi connection. You’ll need to disable the WiFi for the system to latch in on the cable connection. If the speed is not different then your ISP needs to service your connection as that is the problem!

If using the Ethernet connection is better then your issue is something with the router or It could be as simple as a configuration issue. Open up your Network control panel and unlock it so you can open up the advanced options section. Here you will see the cached WiFi networks your system has connected to. You’ll want to slide your routers AP name to the topic it isn’t there now, and you should delete any that not belong. Safe the settings and exit then restart your system didn't that fix things?

Let us know what you discover!"