SUPER SLOW INTERNET Belkin n750 dual band router?

[Sporadic]

Skip wireless for the time being, and sort the problem wired first.

As it works with the laptop plugged in to the modem, you know it's not either of those. Now wire (not wifi) the laptop to the router, and the router to the modem. If that works, it's not the router, if it doesn't then it's the router.

The next stage (assuming the router works properly on the wired connection) is to try the wireless. Ideally you need a least two devices, or one laptop and two wifi dongles. This will help to see if it the device's wifi, or the router which is the problem. If both devices have trouble, it's the router, otherwise it's the device.

In this situation, you want to test the set up, change one part, test again, change another, test, and so on.