Hi, i have a dell Optiplex 7040 which i have installed a working RouterOS
This machine has an M.2 NGFF slot available and i want to add a wifi card on this slot, to use as a wifi interface on RouterOS.
Do u have any idea which wifi card should work ? I tried to look a for a hardware compatibilty guide, but there was none !
Your time and help is appreciated

x86 ROS have same drivers as other platforms, so this means only specific Atheros and Qualcomm radios are supported.
To list kernel modules in routeros-xxx.npk use 7zip to extract it and you get following kernel modules:
AR5211 AR5212 AR5416 AR9300 AR9888 QCA9888 QCA9984 WIL6210
You can then check wikidevi to see what cards in what formats are available for these radio chipsets. But I'm not aware of any in M.2/NGFF format, ARxxxx are miniPCI and QCAxxxx are all miniPCIe. But that's not a big deal, you probably have empty PCIe slot as well and converter card from PCIe to PCI is cheap on Aliexpress. So it's more about finding suitable card... look at ebay/aliexpress/etc
If you are fine with just 802.11n, your Optiplex have single PCI slot, used AR9300 + converter card can be found 10x cheaper than 802.11ac QCAxxxx cards. Older miniPCI cards like R52n are really cheap nowadays... and 802.11n is enough for 2.4GHz anyway... same for miniPCI to PCI cards, very cheap as almost no one uses these anymore... so that might be easiest way to get started, then you can later add QCA chipset card if you manage to get one.