Thanks ryane77 for you answer.
This SIM card is having no issues in the other dual SIM phone (not Apple) that I have. My conclusion is that the issues are with the iphone SE and not with carrier.
Once I activate the line on this SIM it searches the carrier and finds it after a while (to me it seems longer than before). I can't say exactly how long after but surely in less than 30 minutes I can't see any carrier for this SIM any longer and it says "no service". It will never connect again to the carrier itself. I have to deactivate the line and reactivate it again. Than it starts all over again.
I think it has to do with the roaming. This second SIM is from another country. The service must go over roaming. Roaming is not a issue since in the other phone the SIM is working w/o problems.
I tell that because if I use a second inland SIM card in the iPhone SE there are no issues.
So you would say, the issue is the SIM card itself or the carrier but why I have then no issues in the other phone?
Thanks and regards,
Stefano