Need to purchase a dual band wifi card for Inspiron 7459

The specs say: One M.2 slot for WLAN and Bluetooth combo card

Have you tried rebooting the router and updating the WiFi driver? Or changing the 2.4 GHz channel being used by the router and PC?

Depending on which WiFi card is installed, and the version of Windows, you can find WiFi drivers on Dell's support page for this PC. Look in Device Manager to see exactly which WiFi card you have and be sure to select the correct version of Windows on the support page before downloading the driver.

Since this is an All-in-One, it might be difficult for you to swap WiFi cards. So perhaps a dual band USB dongle WiFi would be easier to use, and it has the advantage that it can be moved to another PC, if/when you want.

If you go with a USB WiFi dongle, you'll have to go into BIOS setup and disable the internal WiFi card before you'll be able to use the dongle. But keep in mind this will also disable Bluetooth on this PC, so you'd have to buy a WiFi/BT combo dongle if you want to connect any BT devices to the PC.

Also keep in mind that dual band WiFi uses 5 GHz as the second band. The 5 GHz frequency may be less crowded in your neighborhood which could improve things, but 5 GHz typically has a shorter range, so the 5 GHz signal could actually be poorer if the router and PC are in different parts of your house.

If the PC and router are far apart, try to move them closer together. Or maybe consider a compatible WiFi range extender that you plug into a wall outlet closer to the PC that amplifies the WiFi signal from the router. That way you don't have to mess with swapping WiFi cards in the PC or USB dongles...

Sure your router is dual band? Does it support WiFi 802.11 ac?

Edited

I agree it is an M.2 card and might be described as a PCIe Half Mini Card /M.2(NGFF).  I have linked to something similar on Newegg and yours is the one on the right.  Make note the two antenna connectors are on the same side.  The other one is what would have expected.

If you can find an Intel 7260 ac in the correct configuration it should work well but without an AC router as Ron mentions it may not help over an N version.  I would stick with Intel, especially if you want to use WiDi..

www.newegg.com/.../Product.aspx

Take the card out and use an Edimax EW-7811UTC AC600 Dual-Band USB Adapter.  $20 and works fine.

https://www.amazon.com/dp/B00FW6T36Y

You don't actually get full speed without a wave 2 dual band router however.