Did you request a change? What is the exact make and model of your phone?
TrueMove H does have a 2100 MHz (3G:voice and data) service, and they re-sell CAT 850 MHz 3G (voice and data).
I'm honestly not sure which service you've been moved to?
You should disable data.
Many AIS and DTAC customers are currently transitioning to the new 2100 MHz subsidiaries. I am on DTAC and was moved last week.