I arrived at Siem Reap airport last night and bought Smart's SIM, featuring 5.5GB data capacity for only $5 (SurfLikeCrazy plan). It surely passed the connectivity test (by turning off WiFi), and the cellular sign on my iPhone 5s shows it catches 4G wave. Great, great. But it went stop working in downtown Siem Reap. Wave strength sign says it's 5 out of 5. Hmm, strange...
I'm using old school iPhone 5s and tested some ways, found that tuning "Voice & Data" from "4G" to "3G" worked fine. I'm skeptical whether this data plan is not for 4G LTE or not. It's OK even under 3G, but would like to use 4G if available... Hmm, this guy says SurfLikeCrazy did work for "$5 for 5.5GB" plan.
SurfLikeCrazy introduction also says it's also available under 4G. Oh, it also noted "APN configuration: smart". I left untouched APN settings that I have used in Japan (IIJmio).Impressed with Smart Cambodia mobile service. $5 for 5.5GB of 4G speeds (about 20mbit)— Ryan Slade (@frefity) October 15, 2015
In the end, removing "Settings" → "General" → "APN" made me happy. It made me confused that 3G worked fine even under the former APN settings.
Do not forget to restore the former settings when you're back to your country.