This morning the WiFi connectivity problem had spread from iPhones and iPads, which couldnt connect at all (but could connect using a phone personal hotpost from cellular sourced data), to our Macbooks, which could connect to a few websites but not all (error server not found on Safari or site unreachable on Chrome). I phoned Apple and eventually determined that the phones and Ipads had self assigned IPs (the laptops didnt). They told me to phone my ISP and get them to refresh the modem remotely (we had already tried that multiple times at home with no effect. Also tried all the suggested fixes on this thread: "Forget this network" and re-joining, toggling airplane mode on/off, turning off "private connection"--all with no impact). After the IPs were reset by the ISP, problem was solved on all devices. I still dont know if this problem originated with the ISP or was an Apple software update issue--but I find it odd that it happened precisely when all our devices updated (Big Sur 11.1 and iOS 14.4). But anyway click the (i) by your wireless connection and see if the IP address starts with 169.254. If it does, it is self assigned (no idea why or why that matters) so try calling your ISP and asking for a refresh. Good luck. Literally a full 2 days of my life that I will never get back working on this.

Starting having this problem on 14.2 two days ago and figured it was because iOS needed to be updated, so have been trying nonstop but cant connect to wifi long enough to get it done. Tried the trick of turning off private connection, no dice. I got a couple of the devices to update by making a hotspot from my cellular from phone (burning my data). But cant get the other two to work. I am not a computer programmer so have no idea how to manually set DNS as some others have suggested. The whole reason I have Apple products is because they are (usually) more intuitive to use than Windows. It's now snowing buckets and the kids have a 5 day school break, and now we have no devices. Thanks a lot, Apple. I dont care about all the bells and whistles you added to this update--I will never use them. But I work remotely and my kids use their devices for distance learning, so the entire reason we have these devices involves connecting to WiFi, and you have now made that impossible. Epic fail!