No Sorry, I cant find any fix, or anyway way to fix it.
I have a neighbour who purchased an exact S5 Mini (on my recommendation) from same supplier, all brand new SM-G800F (gold) and his holds and saves local wi-fi password as it should on 6.0.1 (Marshmallow) with my router too, he can toggle off (switch wi-fi) and then back on and it auto connects like it is supposed to do.
But alas, mine still wont Auto Connect if I toggle wi-fi on or reboot phone at all.
From what I have read online it is a fault that can only be rectified by Samsung replacing the handset.
Then I heard one guy who got his to work by connecting to a wi-fi at another location, then he came home and from that point is 'auto connect' worked on his router???
I tried this and it did not work for me, I have copied and triple checked ALL the settings on my neighbours new 800F which works on my LAN, but my own 800F will not.
Funny thing IF I am connected to my LAN (Router) and then turn off router for say 2 minutes and then turn Router back on the Phone will AutoConnect, but wont Auto Connect to wi-fi IF phone is turned off or wi-fi is toggled off and then back on.
Clearly Phone is not 'holding' the saved passwords of my LAN, and proof that it is not my LAN/Router as neighbours 800F works as it should.
I have run out of ideas, just a dud phone for this task...