1800 MHz FDD Band 3 (for Mobile LTE, Mobile devices - Dialog and Mobitel provides )
2300 Mhz TDD (for Fixed LTE, Only dialog / Lankabell providing, SLT is under planning).
So, if you're choosing a phone, you must go for 1800Mhz FDD.
FDD means frequency Devision Duplexing.
If you purchasing a LTE phone from outside sri lanka, you have to be more careful with FDD and TDD (Time Devision Duplex), Coz some countries use 1800 TDD for Mobile LTE as well.