I don't think Iran is as oppressive a regime as you make it out to be. AFIAK the act of using (or attempting to use) Signal is not illegal. The resources required to trace an IP and track down the user simply because they used a messaging app seems unfeasable. Not to mention IP addresses on mobile devices are highly dynamic especially if you're jumping between wifi and cellular. Correct me if I'm wrong but I think in practice it's pretty hard to link IP addresses to actual individuals.