So the solution of going into your router and binding your Galaxy S8 to an unused IP address is definitely the answer for the majority of these issues. However, be aware that there very well may be two places where it appears that you can bind an IP address to your device's MAC address. For example, on my Netgear router under Network Configuration>LAN Settings>LAN Groups you can add a device to a LAN group and assign it an IP address for its MAC address...there then is an option to save that binding. However, there's another place under Security>Address Filter>IP/MAC Binding where you can explicitly add a device and assign it an IP address for its MAC address. This IP/MAC binding page supersedes the LAN Groups page. So if they don't have the same IP address in both places for the device's MAC address, then you will still have issues with that device fully connecting to the Internet through your router...
Same or very similar problem on my S8+. I'm using a Tenda MW3 mesh network to which all of my other devices connect just fine. However my S8+ will not switch nodes no matter how much stronger the signal compared with the one it is currently connected to. And for some reason it almost always wants to stay connected to the base node, and will switch to the 2.4ghz band even when the 5ghz signal is perfectly fine. So when I'm in my office, which is through a couple of walls and some 30ft from the base node, it stays connected to that node on 2.4ghz even though the signal is terrible. Then I get the ! like everyone else, and it still doesn't want to switch.
edit: forgot to add that if I turn wifi on and off, it will connect to the closest node properly. But as soon as I'm closer to the base node it switches to that and won't let go.
So far I have tried resetting my network settings, but this had no effect.
I have the same disconnect issue except I get no indication that I have no internet. There is no explanation mark. I managed to reach 1/3 of my mobile data in 1 day with this issue. No other devises have issues stating connected to internet while on wifi.