You can purchase an Unlimited data @10Mbps pack from AIS, Truemove H or DTAC for 481.50 per month or 100GB of 'Max speed' data for 399 baht per month.
Alternatively, they all have variations on this package for 300 baht monthly(This one from AIS):
5G internet 40GB (+15Mbps internet speed 60GB per month)
FUP 1Mbps
Unlimited free calls to all networks (15 minutes/call)
AIS WIFI free unlimited
AIS Play family free