If you've ever had a Mac with a Wi-Fi hardware problem, then you know that attempting to use a USB dongle to fix your Wi-Fi has long been a funky experience. That's especially been the case over the last couple versions of macOS. Thankfully, Edimax has released an adaptor that not only works on current and previous operating systems, it might even work in the future.
The adaptor is the Edimax Dual Band AC1200. It costs around $50. As an adaptor, it's fine. It supports 802.11n and 802.11ac, has speeds up to 866Mbps, and packs in plenty of modern Wi-Fi tech under the tiny hood. It works with Linux and Windows. What's nice here is that it also works with macOS, including Sierra and El Capitan.
I'm sure that out of the millions of other Wi-Fi adapters out in the world, there are a few other ones that actually work on macOS Sierra. But I tested several out recently and had no luck until finally landing on this one, so consider this a PSA, as opposed to a recommendation of the "best" option for anyone dealing with this issue.
When it comes to using the adaptor, it's pretty straightforward. As has long been the case with Wi-Fi dongles and macOS, you'll need to install a driver and reboot your computer in order to use the dongle. In this case it also installs a weirdo utility for managing your Wi-Fi connection, because you won't be able to do so from Apple's built-in menubar Wi-Fi utility. Regardless, the driver and app gets the job done and is at least a little simpler then earlier methods that required you to launch Wireless Network Utility.
I've had to use this style of USB adaptor for a number of Macs over the years, if not for myself than for troubleshooting other people's busted MacBooks. I don't know why, but all of my friends seem to have trouble with the built-in Wi-Fi on MacBooks after a few years. For some reason, using a third-party USB dongle on Apple laptops has been problematic for a long time and often requires weird workaround because of driver issues. While you'll still have to deal with installing third-party drivers, the Edimax Dual Band AC1200 isn't as obnoxious as previous solutions. The software makes sense to use, works fine enough, and most importantly, can drastically improve the Wi-Fi connection on older Macs.