Cellular & WiFi connectivity issues since going to iOS 18.1 on iPhone 15 Pro Max

Do you have a 3rd party VPN installed? If so, delete the VPN profile in Settings/General/VPN & Device Management/VPN. Don't just turn it off; delete the profile. If that fixes it, you can try reinstalling VPN.

The reason the first thing to try is deleting the VPN profile (as well as the VPN app):

An important and frequently overlooked fact is that when you install VPN it configures itself to the specific hardware, iOS version and network that the phone uses. If any of those change it can break the VPN. So any time you change any of these, including an iOS update, you may need to delete the VPN app and profile, restart your phone then add them back (if you still want to use VPN) so it can configure itself to the new environment.

But the fact is that VPN (Even Nord VPN, which is better than most) reduces your iPhone’s security, rather than the myth that it will improve your security. Here’s more detail→VPN: What you need to know - Apple Community

Yesterday start to present the same problem of this person on the top. iPhone 15 Pro Max: Took the upgrade to 18.0.1. Then, about 2 weeks ago, we took the upgrade to 18.1. Not good. Since then, cellular and WiFi connectivity is all over the place (it was a bit wonky on 18.0.1 but not as bad as 18.1). The phone often doesn't automatically reconnect to our home router when we get home. When we are out, the phone won't connect via cellular. We weren't able to connect to free WiFi at a doctor's office this morning. We can text and make calls and receive texts and calls. We even get the notifications that email has arrived, but we can't look at the emails nor use any apps. We reset the network settings and that seems to help for a little while. We have toggled cellular off/on, WiFi off/on, rebooted the phone. There is no consistency other than TOTAL FRUSTRATION. We suspect iOS 18.1 has bugs in the connectivity area since we had NO issues with this when on the iterations of iOS 17. We traveled out of the country with the phone on 17 and used free WiFi and an Airalo eSIM with zero problems. Apple, you have to fix iOS 18.1!!!!!