Just because a device cannot run Apple Intelligence doesn't mean the end of the road for updates for that device is iOS 18.0. There is clearly an issue here.
What shows in the beta updates in Settings > General > Software Update?
You say "the system does not upgrade to 18.1", but why doesn't it? Is the 18.1 option shown but it fails to update? If so, what's the error? Or, is the 18.1 update just not listed?
Honestly, you must give us the information necessary to answer your query. Saying there's an issue - but not telling us what the issue is - means we have to ask you, so it just delays everything.
Remember, if you're on the iOS 18.0 beta trail you'll be offered iOS 18.0 updates. If you want to use 18.1 you have to pick 18.1 as the update path.