According to the TrueMove H support pages the APN settings for Real Move/2100 LTE are a bit different from CAT/850.
APN type: default,hipri
Authentication type: none
I assume that if you have subscribed to a LTE plan (iSmart), for which there may be a 99 baht/month surcharge. This surcharge may be waived for some promotional period.
I assume you have to enable roaming so you can roam onto the Real Move 2100 MHz 3G network, and the CAT/850 MHz 3G network?
Again, with a properly provisioned plan and service from Real Move, coverage, a properly configured phone you should be able to get 3G/LTE. It seems like you have the most current baseband for your model, and one assumes it can interwork with Real Move's 4G/LTE.