I have had the same problem of "data roaming" when I have used a SIM card of an operator that is operating in another operator's network. In other words, they don't have their own network but they "rent" another operator's network in which they operate. "Roaming" means that you use some other network than your own operator's native network. I am sorry I don't know why the SIM 2 gives you "Emergency calls only".
Thanks for your reply. I understand what your saying. I'm using a Galaxy S6 (posted there but got moved here). Maybe it's not a problem isolated to S6.