The one time I ever had Wifi issues was early last year. Same symptoms. Turned out to be the modem my ISP uses. It's a modem that includes a built in router and wifi gateway.
Anyway, both Windows 10 and El Capitan were having connectivity issues since I upgraded things to those releases. I upgraded the OS X devices first. Figured it was a problem with the Macs. Tried everything. Gave up. Upgraded a PC to Windows 10. Had the same problem. Tried more stuff. Nothing fixed. In a last, desperate act, I contacted my ISP and had them put my modem/router into bridged mode. *poof* Problem was solved.
I'n not saying it was the OS X and Windows updates that caused my problem. It's quite possible that my ISP pushed out a firmware update to my modem at roughly the same time I upgraded the first OS X device. But, since I tried everything else that I could try, the only thing left for me to do was remove the one device remaining from the equation (or at least reduce the technological footprint.) A device over witch I did not have complete control. My cable modem. So, that's why I contacted my ISP.