SIM card both detected and not detected on Samsung Galaxy s4!!!

[AC Question]

So i have a strange issue with my SIM card on a Samsung S4. When I try to turn on mobile data, I get an error saying "No SIM card detected." But heres the strange part: when I go to my apps, an app has appeared saying "Orange Services" in the language of the Orange store where I got thr SIM, with a logo showing a SIM card. So why do I get the detection error? My OS is the latest version, and i have also tried a cache clear in Recovery Mode, which didnt work. Unfortunately I bought this phone used from a UK seller so its not on a warranty, but its supposed to be unlocked, though the original network was O2. I am also in Warsaw Poland if that helps, and I got the card from Orange in the same city. If anyone can help me out Id appreciate it! This is also my first android phone so Id appreciate patience and clear instructions too. ☺ Thank you!

 

[B. Diddy]

Welcome to Android Central! What's the exact model number of the phone? I'd want to make sure it fully supports the network bands that Orange in Poland uses.

Have you tried getting a replacement SIM?