I extended both my sims at the AIS shop in Terminal 21 , there is a top up machine , you just keep throwing 20 baht bills in it and the whole 20 baht is added to your account and also extended 1 month ,

You can only extend up to 12 months , I assume you could come back in 11 months and do it again ,

And the staff cannot do it on their tablets , they were the ones that showed me how to do it at the top up machine , one month at a time !