I'm facing the same issue with my Dlink Covr 1203.
Turn out if you are using an android mobile, you will face this issue with 5ghz which are the high speed that you are getting from your ISP.
Unfortunately there is no fix from DLINK to this moment on any covr since you can define the channel for each speed.
I got EXO AC2600 and i'm able to get the full speed of the 5ghz on my S8+ / Around 150 mbps on speedtest
However no matter what i do, My S8+ will always connect to covr on 2.4ghz.
You can check that from Management -> Statistics page and you will see that the 5Ghz will not be called at all. I was about return it back to shop and call it faulty until i called a friend (who own an iphone) and asked him to test it, Guess what, it work perfectly for him. So i figured out it's an issue in the software or the firmware
Very SAD that DLINK made such a product that you can't even control the 5ghz in it
PS: Even after enabling the DFS Channel, it's still not doing anything
My suggest either buy something else that support channeling configuration or get a real router and connect it with LAN cable then you will get a better control over your network
