I connect to my D-Link wifi now but can't connect directly modem via cable

I have run into this same issue and found a working solution, hopefully it will work for you. When this happened to me it was an issue with how windows configured the interface metrics. These are basically values which tell windows which connection to use when multiple are present.

First, click on the start button and type ncpa.cpl, which will come up with a control panel item. (This is just a shortcut to get to the connections list. You can also get to it by clicking 'Change adapter options' in the network & internet settings)

Here you will have at least two connections, one for ethernet and one for wireless. It is possible to have more than that, but we are not interested in those right now. You can check for the simple fix here by making sure ethernet isn't greyed out. If it is, right-click on it and enable it, which could very well be the solution to your issue and you can stop here.

If that is not the case, you can try changing the interface metric for both the wired and wireless connections. This setting is buried pretty deep, as it is not something which usually has to be changed. Start by right clicking on your eternet connection and selecting properties. There will be a box with a list of items. Find and click Internet Protocol Version 4 (TCP/IPv4) (click on the words themselves, not the checkbox) and select Properties under the list. At the very bottom of this new window click the Advanced... button. 

You should now have a window titled Advanced TCP/IP Settings with very little information in it. At the bottom of this window uncheck the box showing Automatic metric, this will make the text box below useable. For the ethernet connection, put in a value of 2, then click OK on all of the windows that have been opened until you are back to the Network Connections window (the last window will show Close instead of Ok.. just pretend it says Ok).

Next follow the same path through the settings, this time starting from the wireless connection. Once you get to the interface metric, enter 3 this time. What this should do is set your eternet to have a higher priority than your wireless connection.. Windows should do this automatically, and there's a chance your laptop's BIOS should disable wireless when there is a wired connection, but for some reason none of this is happening. The reason we didn't set ethernet to 1 is because 1 is for the system's loopback service, which is fairly technical to explain and not something we want to mess with.

Although this solution worked for me when I was having a problem similiar to the one you are having, there is a chance this will not work for you! If that is the case, I suggest following the same path of options for each connection once more, and rechecking the box next to Automatic Metric, as this setting can complicate other troubleshooting.