Where you in a different province? I deposited money at the counter into my account once in Chiang Mai (my account was opened in bkk) and they charged me a commission. I almost lost my mind, charging you money to give them money is fking insane to me.
You don't need a workpermit for a post paid plan. Whoever told you that is a moron. Also they don't throttle speeds for foreigners, they do for pre-paid plans. And every Thai person I know is on a prepaid plan because it doesn't require a commitment.
You're probably on a tourist sim though. You need to go to an AIS shop and get an actual plan, that gives you fixed data speeds. I have