DATA SIM cards offered online

[Patanawet]

I really don't understand the plethora of advertisements I'm seeing at the moment for Sim-cards from sellers such as Lazada. 

They seem to offer one year unlimited data. The prices seem to be about 1200 Baht. What do I get for that? Are there additional monthly costs?

Can I use it in my second SIM slot on my phone whilst keeping the other one for my original SIM with my phone number?

I am, at the moment, on an AIS Mao Mao plan for about 500 Baht per month giving me unlimited data at a fair speed (just checked and I'm getting 12.4 Mbps). I rarely use conventional phone calls but do send a fair amount of SMSs.  I did search earlier threads but really didn't understand much. Thanks in advance.

[Eindhoven]

They give you what they advertise. Likely 60 GB at whatever speed is available at your location per month No hidden costs. 

 

If you want to stick with AIS, you can get unlimited 4 Mbps/10 Mbps for 150/200 baht per month. 

 

Truemove H offers a similar package for 1750 baht for the year.

[Patanawet]

Thanks Eindhoven (or dank je wel????).