I have tried all the solutions to try to fix this, and none of them work. I cannot understand how Samsung are allowed to ignore this. Spending over £1000 on a phone, only to have it rendered virtually useless by Samsung's indifference towards their customers is criminal. I have always praised Samsung, but this is the last product of theirs I will EVER own.
OK. After much confusion with the support guys who could only tell me to submit my phone via courier to be fixed, I had a play myself. What fixed it for me was to go into my router configuration page and carried out the following changes. This is for a plusnet router but other routers will have a similar facility. Remember to backup your present router configuration before changing anything to be safe.
Select Advanced Settings and enter your Hub admin password when prompted (unless you've changed it, you'll find the default password on the settings card)
Continue to Advanced Settings.
Tap on Wireless.
Choose 5GHz tab.
Change 'Sync with 2.4Ghz' to No.
Change the name of the 'Wireless SSID' to one you will remember. All I did was keep the original name but add 5GHz on the end to distinguish it from the 2.4 GHz setting. I left the wifi password as it was originally.
Save the changes.
Then, on your phone forget your old Wifi connection and select the new 5GHz one you created.
I add a photo for clarification. I have had no lost wifi connections at all since doing this, and when I disable wifi and re-enable it, it connectis immediately without fail. No mor waiting and constant attempts to connect and running down the battery. No more Wifi Authentication errors. I hope this helps others with the same problem.