I was going to post this on a Samsung(USA) site where I found a thread, but it was so bloody awkward to create an account / sign in that I couldn't be bothered to complete the process.

Anyway

(SOLVED for me - posting just in case it helps) 

I've been having the wi-fi "internet not available" issue for a solid week now on my UK Samsung Galaxy S7.  Won't stay "connected" for more than a few minutes usage.  All other devices seem fine.

The problem appears fixed (or at least the phone has remained happily working for several hours) after I:

Firstly:  Went into settings -> apps -> (...) -> show system apps -> Bluetooth Share and first force stopped, then cleared data.   (I found this suggested as a solution elsewhere.  It did not appear to fix it for me - but I can't rule out the fact that it's involved.)

Secondly:  Went to my wi-fi router's config and DHCP-reserved an IP address for the S7's MAC.   I was careful to pick a *different* IP address from the one the phone was currently using.      Rebooted router (which was required to apply the reservation), turned off the phone's wi-fi (I was actually connected via a different access point, not directly to the router), turned the wi-fi back on once the router was available.

I confirmed that the phone had picked up the new, different IP.  No problems since!

Cheers.