I have the same issue with a Galaxy S9. From time to time I get the "!" and most of the times I have to deactivate and reactivate the wifi to solve it. I haven't seen this issue to appear while connected to the 2.4 GHz network, only to the 5 GHz one.
This is getting really frustrating.
I even ended up changing the router the ISP gave me, but seems like the problem persist.
Samsung needs to get this sorted out now. I've been having this happen the last couple of months and I have been vigilant to make sure nothing was being downloaded while not connected to wifi. I believe the last Jan 20th update happened while wifi was disconnected and so now I am over my data limit. So where do I send the bill Samsung?
I've tried all the various fixes recommended in this thread and frankly, I've had enough. This is Samsung's issue and they need to get it taken care of rather than having their users jump through ridiculous hoops. Supposedly there has been an update for this issue released in various Asian markets......so maybe put in a little overtime and get it out to the rest of us. I'm tired of this S8 costing me money because Samsung can't get their act together.
Having this issue with my Samsung Note 9. This started when I was on Android 8 Oreo and persists even on Android 10. I have tried allocating a fixed IP to my Note 9 and this still doesn't fix the issue of always having the exclamation mark eventually when connected. Any other ideas? This device is the only one that has problems. My old phone and all my family's devices work just fine.