iPhone 5 LTE not working

Ok, it's like this: I have an iPhone 5 since August of last year. I never updated to iOS 7 until now because I was kind of afraid, due to many reasons (reviews I read, battery life, UI, easy-use issues, etc.), but I did it, because, you know, I couldn't use iOS 6 forever and, mainly, I wanted do enable LTE (actually, it's 4G, since I'm from Portugal and there's only 4G here, and don't worry, I know it's different, but I'm only being name-wise, but moving on). I updated last week, and I was kinda satisfied with everything, and I got 4G. However, like 2 or 3 days later, it just stopped working. All I got now is the 3G logo. I try to enable 4G in Settings and even tried data roaming and everything, but 'nada'! I tried to contact my carrier (TMN, aka MEO) and tech support and they told me it could be 5 things: there was no 4G coverage in my area (impossible, 'cause my friend in town has an iPhn 5c and he gets 4G, full signal), the phone didn't support the frequency used for 4G networks here (impossible, again, since I have already connected to 4G), the phone may have an antenna problem, SIM could be damaged or there could be some kind of error in the update to iOS 7 and to try to reset everything. I have warranty, btw.
Someone help me out here? I'm going crazy!