There is a FB group for long-term contract for condo. It's much cheaper than Air bnb. Don't book a whole month before you come here. Just book a hotel or air bnb for a couple of days and look for the condo by yourself. The real estate agent only charge from the owner.
If you see about 8-10 condos, there should be some place you want to stay.
Stayed at Astra Condo near Maya Mall. Modern studio with fast wifi, gym, pool for 12,000 THB/month. Perfect location - walking distance to everything in Nimman. Highly recommend for 1-3 month stays.