BTW it’s not necessary to use an external device for network scanning:
As an added benefit, you see what your computer sees, rather than another device, which for a variety of reasons may perform differently.
Comparing your computer’s readings with those from another device can also give you further insights about problems that may be specific to the computer.
If you’re running Windows 10, make sure Wi-Fi Direct is disabled: start a search for Project to this PC and set it to Always Off. If that fixes the issue, then it means your adapter isn’t fully compatible with Wi-Fi Direct.
Second, I’d check if an updated driver is available for the Wi-Fi card, or a manufacturer supplied driver, the latter especially if your machine has been upgraded to Windows 10 from a previous version. Not all drivers provided by Microsoft through Windows Update are of the same quality, unfortunately, and can malfunction under certain conditions.
It is also possible that your computer’s Wi-Fi card and the router do not work well together because of some specific setting wasn’t available on your old router but is on the new one, like the computer’s Wi-Fi card doesn’t work well in certain modes.
Issues like this can sometimes be solved by checking both the router’s configuration panel and your card’s driver settings, which you can do by opening the Network and Sharing Center, then clicking on Change Adapter Settings and finally Change Adapter Settings. Proceeding by intuition, or by exclusion, you may be able to hunt down the setting that’s responsible and change it accordingly.
If you can’t trace it down to anything specific, you still have the option of running the Windows troubleshooter, which usually fixes malfunctions; I’ve even seen a case of a computer that had similar sporadic Wi-Fi malfunctions that would go away with the troubleshooter and eventually disappeared with some update.
As for your router, it is also possible for newer firmwares to introduce bugs, so downgrading may be worth a try.