What is the distance between your ONT and the router? If it's more than 80 to 90 meters, you won't get 100 Mbps even if you are provisioned for that speed. There will be a degradation.
Your router issue is mystifying though. With that D-Link, you should be able to get at least 90% of the speed that you are seeing with a wired connection. Check if there is channel interference with a wifi analyzer app. Choose a channel that's well away from whatever your neighbors are using and away from cordless phones. Avoid "auto" settings on the router.
Your router issue is mystifying though. With that D-Link, you should be able to get at least 90% of the speed that you are seeing with a wired connection. Check if there is channel interference with a wifi analyzer app. Choose a channel that's well away from whatever your neighbors are using and away from cordless phones. Avoid "auto" settings on the router.