iPhone 4S 3G Signal Issues

Hello,

I've acquired an iPhone 4S second hand from a guy on "local craigslist". It was locked to my network - Vodafone Romania - now the phone has been unlocked but the problem still persists.

All in all, the phone presented itself in very good condition.

Here comes the problem. We all know the 4S isn't 4G capable, that's not the issue.

The issue is with switching between 3G/2G. In areas where there IS 4G coverage, switching on 3G will give me 3-5 signal bars, but most of the calls won't go through, SMS's will fail sending and the internet won't work. Switching to 2G in these areas will make the phone work just fine, except the ultra slow internet.

Whenever I get out of the 4G area, where there's only 3G signal, switching on 3G on the phone will work flawlessly. No problems what so ever. Texts go through on first try, calls as well, internet works flawlessly. I've managed to use 30+ GB of data in such areas (week long vacation and using mobile hotspot a lot).

I've swapped around 15 SIMs. Yes, 15! None seem to improve the situation. Even tried finding old SIMs that do not support 4G, that did not work. I even tried replacing the antenna (internal one. the one with the speaker) and that did not work.

I've talked with the phone company, they say it's a phone issue. Last week I managed to make them unlock my phone from the network. I have to wait around 15 days for the unlock to process. I did that with hope it will solve the problem.

The phone has no marks of having something replaced inside, so the components are 100% Apple, besides the battery which I had to replace because it wasn't holding a lot of charge and I'm not sure it's genuine.

Do you guys have any ideas?

Thanks!