I have bought this in November last year and used in Bangkok. Internet speed is really good. It is even sufficient for use with TV set-top box.
https://www.lazada.co.th/products/i306548374-s533342479.html
The package above include a 4G LTE router and an AIS sim card with Net Marathon plan of 100GB/month for 1st 12 months (ie. 1st year). So, for the 1st year, what I pay is THB2,488. Subsequently, only need to top up the AIS sim card (so far as I remember, to renew another 12 months for the same plan, THB1,790).
Further, the router is unlocked. So, if you want to switch to DTAC, free to do so.