All my apps have stopped working on mobile data since I updated to iOS 18.4.1 on my iPhone 14 Pro Max

None of this is specific to iOS 18.4.1, but are the go to steps for any cellular related issue if you have already verified Cellular Data is turned on in Settings:

• Reset Network Settings at Settings > General > Transfer or Reset [Device] > Reset > Reset Network Settings. This will also erase any WiFi passwords, so you will have to enter them again when you first log in.
• Check for a Carrier Update. - Manually update your carrier settings on your iPhone or iPad - Apple Support
• Force Restart your phone - Force restart iPhone - Apple Support

All apps? Have you gone to Settings>Cellular>scroll to the section titled Cellular Data, then scroll down to Show All. Do you see a switch turned on for apps you are experiencing a problem with? What troubleshooting have you tried? Have you contacted you carrier to see if there is a problem with your cellular data?

The provider should be able to help, or the steps that Mac Jim ID provided about resetting network settings should help.