Having this exact problem. I just bought and S9 for both my wife and I and have them connecting to a TP-link EAP245 which is serving the same SSID on the 2.4GHz and 5GHz bands. The EAP245 is then wired to a router running Tomato. My wife's phone has a static IP configured and she has never complained. My phone's IP is still dynamic and it randomly does this. Especially when I go to the basement where it is likely to switch from 5GHz to 2.4GHz. I have to turn the Wifi off and back on for it to work again. I've just set my S9 to also use a static IP. Hoping this fixes it, but you know I really shouldn't have to. My old S5 and S7 both work perfectly. Samsung, get your s**t together.
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