Hi,
I have exactly the same issue. I have one physical SIM and one eSIM and before updating from iOS 17 to iOS 18 I never had any issues. Now the eSIM has no cellular data at all. It displays 4G next to the provider's name, but no internet.
I first thought this might be caused by the carrier or will be resolved after some days - that they need to update something on their end. I was in contact with them and tried many troubleshooting steps. At first I tried the common steps like re-starting, resetting all network settings etc… after nothing worked I even deleted and re-aded the eSIM. Text messages still go through but no cellular data. I was looking for answers online and found that quite some people also have this issue (especially on Reddit). It all seems related to the iOS 18 update. Hope that either someone comes up with a solution that works (I really tried almost everything except factory reset and/or downgrading back to iOS 17) or that they release a hot fix very soon.
Best regards, Simon