iPhone 12 with IOS 14.2.1 WiFi connectivity issues

What kind of wi-fi connectivity issues? When you say you have to disable wi-fi to connect to websites, it would seem it appears you are connected to wi-fi but cannot access internet? That would be an issue with your router. Try forgetting your network, then go to Settings>General>Reset>Reset Network Settings. Then unplug your router for 30 seconds. Plug it back in, then after it has finished connecting, go to wi-fi on the phone and try connecting to your network again.

We are also having the issue with wofi. Dropped calls, not receiving texts, and not able to call out on wifi. We went to apple and they recommended a new router as well. The routers seem to not be compatible with iphone 12

Try turning off the Private Address function. It is under Settings>Wifi and is a toggle in each network you are connected to. Try turning it off for your home network. It worked for me. You will get a security warning from the iPhone software but if your home network is secure I don't believe there is a real risk. Apple peeps please weigh in if I am wrong.

I have found that the latest updates from 14.5 and 14.5.1 and 14.6 wouldn't even download because it would not recognize the phone was connected wo WiFi - However, Apple support and my phone when completing a "What is my IP" online showed my internet provider of my WiFi Modem/Router. I have swapped 2 devices now and been to Apple Store to troubleshoot. I have seen issues; with their latest software/hardware and the phone does not apparently like when your switch or router combines WiFi signals 2.4GHz and 5GHz signals together and let's the device pick - this appears to have happened with the 14.5 and above updates where this confuses the device and or just wont work at all. This is probably something in the Software or Hardware of the iPhone and the WiFi Adapter it uses - Again I would suggest that Apple needs to probably do some QA/QC analysis of it's phone's since there are many others out there with a similar issue to mine. I'm not saying this will fix anyone else's problem but; this fixed mine.

I also reset my Asus RT-AX88U router back to default and reconfigured it from scratch, because I had it integrated with my Amazon stuff; but now it is using my 5GHz WiFi just fine.

I should also clarify that I am on Xfinity Mobile, iPhone 12 Pro Max using 14.6 iOS

I had this problem at our second home with iPhone 12 IOS 14.6 and two iPads.

Somewhat counterintuitively, splitting out the 5mHz channel in the Xfinity router to a separate SSID allowed me to connect.

I had this same problem and finally took the phone to the Geek Squad after several calls with Apple and Verizon. They had it fixed in 2 minutes. Delete any VPN you app you have. It interferes with the connection.

A new router fixed my issues. Apparently older routers do not work with iPhone 12. A nuisance and expensive fix that shouldn’t have been required

RickVT I turned off Private Address function & it worked

You cannot downgrade iOS on the iPhone. I don't have the Max, but I have an iPhone 12 Pro running iOS 14.2.1 and it will connect to wi-fi just fine. Have you tried connecting to another wi-fi network, such as a public wi-fi, or a friend's?

No this is device-specific ie it’s the 12 pro max and 12 lineup that’s the issue; I and others have multiple apple devices (for me iPhone X and an iPad Pro that work just fine on the same router.) My 12 pro max wifi connection issue is worsening daily, none of the fixes work, even disconnecting from WiFi doesn’t help. I seriously wonder if it’s the modem in the phone.

Bill thanks for the post. You are not alone and you are correct; there is a problem with the iPhone 12 promax and the new iOS. I have a home network that as well set up, my brand new iPad Pro, my MacBook Pro, my iMac, my PC laptop another iPhone 12 pro but not max, my Apple TV and other devices all connect just fine leaving the iphone 12 promax the only device which has an issue. When I try to go to a webpage after waking my phone up there is no Internet connection if I’m connected to Wi-Fi. I can be sitting directly in front of my router yet my phone will not connect to the Internet. However, if I turn Wi-Fi off for half a second and back on then it will pull up the page. Also, if I turn Wi-Fi off it will use cellular data and connect to the Internet. So again I have tested the same process on all the devices listed above and all of them work without any type of flaw or needing a workaround to get to the Internet. 12pmax only

If it doesn't change when you get the new router, then contact Apple Support. It may just be an issue with your individual phone. Mine, and no one else I know with the iPhone 12 are experiencing a problem like that.