After updating the firmware on my Samsung Galaxy S6 not long ago (early December 2017?) I immediately noticed my battery used became absolutely horrible. Like, dead in 90 minutes kinda bad. After doing the normal things like clearing the cache etc, and getting only marginal improvements, I finally noticed that the wifi connection on the phone was having continuous errors connecting to my ATT wireless router- with a message “authentication error occurred”. What? This definitely worked before so why not now? With this info, I did more googling and finally found out a lot of people are having this same problem.
The problem apparently is caused by the ATT router having a 2.4 mhz and a 5ghz radio that both use the same ssid and password- This is how ATT recommends running it so that intelligent wifi clients can automatically switch to whichever radio is giving the strongest signal at the moment.
Something with the new firmware on the S6 broke this capability- likely due to a security patch. SO: The solution, for now at least until Samsung gets around to re-fixing this, is to change the name of one of the radios so that it has a distinct ssid. Then on your phone, you’ll just need to pick which one to use.
I myself did a slightly different hack to get this working for now- I enabled the “guest” radio on the ATT wifi and it only uses a 2.4 ghz radio- so now my phone is connecting to this network instead. This will have to do for now. Battery life is much improved again, I’m curious what my data plan usage will look like for this month though.