After reading several pages of the issue (which I also had), you gave me the best diagnosis to troubleshoot the problem on my phone. My config is: ATT Modem, Netgear Router dual-band, Samsung S9. As soon as I went to the advanced options to try a fixed IP address on the phone, the DNS that was there was my router IP 10.0.0.1, instead of the classic Google DNS 18.104.22.168. If I changed, it worked on the phone immediately (I always had internet, but no access to Google Play, and also the annoying warning). So the problem wasn't the phone, or the internet. The problem was that doing DHCP for some reason the DNS wasn't being assign as expected. So I checked my router config, and I had the wrong configuration in the DNS servers section, being 22.214.171.124 the second in the list instead of the first. Changed that in the Netgear router and immediately both 5G and 2.4G networks worked on the phone. No problems in a week since then.
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