is listed as solved. it is NOT solved. the problem has existed on s6 phones since at least nov 2017 and has NEVER been addressed by samsung. there was another thread on the same issue that was closed. i don't know what it's going to take to get samsung to fix the issue. now i see that other devices are having the same issue
I also have a S6 Edge, a S9+ and a J7, all up-to-date, all failing to connect to my in-laws' TIM router (Italy) when no phone has issues to my Fritz!Box 7590 at home.
S9+ eventually got through for some time then it errored out, S6 never got through.
Taking hint from another thread, I created a simpler guest SSID (6 characters, no dash unlike the default TIM-<8digits> one), still protected via WPA2-PSK, and ALL phones connected flawlessly.