To be honest the Fairtex gym is a bit touristy. You may or may not get some good training there depending on whether someone takes a liking to you. More often you will get pad work and minor instruction.
If you are really serious and don't mind heading outside of Bangkok then
head to Por Pramuk Gym. Not as chic at Fairtex, but you will get better training.
I'll be back in Thailand probably November, and then again late Feb and March for the WMF World Amateurs in Bangkok. All depends on UFC schedules and what not though, I should have been there last month but had to cancel because of a fight in Denver.