iMessage not available on mobile data iOS 14.4
I've noticed today that I can't send iMessage when I go "outside".
When I wanted to disable iMessage which was originally enabled ...
it was not possible to turn it on again ... as it asks to enable iMessage in a mobile data access
but unfortunately, iMessage is not on the list of applications available on Mobile Data as it has been moved into "System Services" => "Messaging Services" - but over there there is no button to enable or disable the service at all.
I found a few similar posts from much older versions of iPhones and iOSes including this https://apple.stackexchange.com/questions/243206/how-to-fix-imessage-unavailable-turn-on-cellular-data-or-use-wi-fi-to-set-up-i which didn't work.