I've had some issues with my old laptop (unibody macbook pro 2009) - and changing the frequency of the router itself can make most the difference. The 2.4ghz and 5 ghz bands don't overlap whatsover, turning one off won't do anything. The most common cause would be that (if you're in an area with lots of wifi) there could be interefence, and someone logging onto youtube a few doors down that's running the same channel you're on could create a lot of noise. 5ghz channels don't have nearly the range as 2.4hz channels too, so you need to be relatively close to the router. I usually use 5ghz channels because the transfer rate (for me) is 140 mbs while the 2.4gz channels are out around 20-60 mbps. But my old macbook gets like 20 mbps from 5ghz and is running a driver 6 years old. (like a boss).
If you 'option' click on the wifi icon it'll upon up a diagnostic tool that let you examine that. If everything is fine, no router issues, and you're certain it was caused by el capitan - slight chance something in the network protocol is configured wrong or corrupted so maybe try redownloading - including the bootloader. But 90% of the time it's network interference and router issues.