Limits on data are a function of the SIM card you buy and the plan that you sign up for, not of the device. In other words, your portable router is also going to be limited by whatever plan the SIM card that you put into has, just as with a smartphone. Swap the Sim cards between your router and the phone and it should be the phone that has unlimited data.
I did discover that T-Mobile in the US throttles down hotspot connections to something like 1 Mbps speeds with the prepaid SIM cards but I hadn't heard if any Thai providers doing that. However there are workarounds for that even if it happens, including an android app that prevents your ISP from being able to tell you have set up a Hotspot.
If handholding during initial setting was a selling point, you could have just as well walked into an AIS store in a shopping mall, told them what you wanted, and handed them your phone, and you would have been good to go too.