[AC Question]

I have a galaxy s6 that just updated to the new Nougat (I think?) And now my wifi won't stay connected for longer than a few seconds before disconnecting and then reconnecting.

I tried switching on and off again. Toggling wifi/airplane mode (was told would help?) and also forgetting network.

[B. Diddy]

Welcome to Android Central! Does this happen on every wi-fi network, or just your own? Try rebooting your router by unplugging its power for 10 seconds.

Also try wiping the cache partition: https://support.t-mobile.com/docs/DOC-29314

[IAmSixNine]

Try going into location settings, then look for improved accuracy with WiFi and Bluetooth. Make sure those are unchecked.
On my S7 Edge and Note 4 a while back i had an issue with those being enabled and my WiFi connecting and disconnecting rapidly. When i disabled that feature my WiFi returned to normal.
That bug also caused my phone to run hot and give me poor battery life.