I have used quite a bit in the past the TP-Link wifi usb adapters - and the models I've tried have not given me any hassle.
I'm afraid I haven't tried yet any dual band one, but the TP-Link Archer T4UH AC1200
http://uk.tp-link.com/products/detai...cher-T4UH.html looks like would be a possibility.
According to this page
http://askubuntu.com/questions/69415...ireless-device it has the Realtek RTL8812AU chipset. According to this page
https://wiki.gentoo.org/wiki/AC1200_...ltek_RTL8812AU this appears to have drivers in the Linux kernel - but you will have to find out when they were introduced, as you might either need to upgrade to the latest Slackware -current to get them, or even compile your own kernel if they are very new.
There is also the cheaper and somewhat slower TP-Link Archer T2UH AC600 - which uses the RA-Link MT7610u/RT2860. Unfortunately it seems that this needs a driver compiling separately - as far as I can tell.
If anyone here has installed and/or used either of the above, it would be interesting to know some real life reports.
I hope the above helps some.