What’ve happened? (Fixing ipad WiFi-connection)

I just fixed my ipad’s problem of unable to access to wifi, but I’m not sure what exactly was happening.

It started with the router was off (apartment power issue) and restarted. Shortly after, allother apple devices in the house (an Iphone and a couple of other ipad) were back online, except one particular iPad. Settings were legit, wave sign was on the top-left screen, but no network connection.

The ISP and router were good, as other apple devices literally inches away were all connecting fine, only that one;

Meanwhile, that particular iPad can even connect to the internet via iPhone hotspot (I didn’t get chance to test whether different WiFi network would work).

Tried reconnect, forget-and-rejoin, restart router, reset device, reset network settings .... none of them worked for the first few rounds. Eventually it was fixed after every attempt above has been done for a couple of times ( I think it end with first reset network settings, followed by restart router).

Does anyone know what was happening during this, I mean, where exactly was the problem located?