Sorry to reply to this old post but I though it’s better than writing a new topic with the same subject.
I have a very similar issue with the 2.4GHz Wifi. Interestingly, computers with Windows do not have the problem. We have a iMac which has the issue very often (in the order of minutes, sometimes seconds) and some PCs with Ubuntu that have the issue less often. Phone with Android also seem to be more stable. The 5GHz Wifi doesn’t have the issue.
Same symptoms: the Wifi connection is stable but without internet connection, even pinging the router with its IP doesn’t work.
I’m ready to send anything that could help resolve this issue.