Why can't I access United Airlines websites on my iPhone 7 Plus?

It is most likely the VPN. I worked with someone last week using Southwest on same issue. Solution was delete VPN software, delete VPN profile, and reset network settings. Once that was done, no problem logging in and adding boarding pass to Apple Wallet.

iPhone > Settings > General > VPN & Device Management > delete profile

iPhone > Settings > General > scroll down > Transfer or Reset iPhone > Reset > Reset Network Settings

VPN's are completely not needed on iPhone and often cause issues. Turn the VPN off and then see if you can get access to United Airlines. Better yet, open Settings > General > VPN & Device Management and remove the VPN completely from your phone. VPN is only useful for people who are required to use one to access secure work or school servers.

Is your iPhone 7 plus updated to the most current iOS version it can support (iOS 15.8.4)? How are you trying to access United Airlines? For example Safari? Do you use a VPN on your iPhone?

Do you mean open their website or log in to your secure account with the airline? Is your iPhone updated with its latest security update? What web browser are you using?