IPhone 5 no 4g after SIM switch

Question

ISSUE (SOLUTION IS BELOW):
I recently traveled from South Korea to the United States. When I was in the United States I switched to a US SIM card to have 4g data service and phone service.  When I came back to South Korea I was unable to get 4g service.  I was only able to get 3g service.  The iPhone 5 is a 64G white A1429 GSM purchased in KOREA.  I was told that my iPhone 5 was broken and that it must be sent back to get fixed.  (See below for how I really fixed it in a few minutes). 
This also occurred on my Verizon iPhone 5 (CDMA A1429 model) that I brought from the US to Korea.  My 4g was working great until I inserted a US SIM card into it; then all of a sudden NO 4g service anywhere.  Of course I was told that this was because it was a US phone and they couldn't help me.  I was also told that many other non-Korean iPhone 5 owners were experiencing this same type of problem.  I bought a new iPhone 5 (from Korea) and experienced the problem above.  So it is NOT an issue of Korean / Non-Korean iPhone 5 (as long as your frequency band supports both locations).
SOLUTION:
I factory reset the phone.  Removed and installed the SAME SIM into the iPhone.  Then I restored the phone from a backup.  This immediately gave me access to my 4G data!!  The cell provider employees were AMAZED that it worked (on both phones). 
RECOMMENDATION:
Make a backup of your phone IMMEDIATELY before doing a factory reset and all that you will be out is the time it takes to reinstall apps & music.  Make sure to transfer purchases so that your apps sync as well.

Answer 1

Many people don't know what to do or more importantly what NOT to do when they get their iPhone wet. Here's what you SHOULD & Should NOT do if it ever happens again.
The Truth About Water Damage
http://iheartrepair.blogspot.com/2013/07/the-truth-about-water-damage.html
As far as the "no service" issue after the phone dried out, that's very common especially on the iPhone 5. I've seen a few iPhones that I've worked on do that. It may be a high risk water damage area on the iPhone since the sim card slot is not waterproof. That pinhole on the side of your iPhone is not a good thing when it comes to water.