The LTE capable Simple Mobile plans include unlimited domestic voice, unlimited domestic and international texting and unlimited data. Data speeds are throttled to approximately 256 Kbps after 3 GB/month on the $50 plan and after 5 GB/month on the $60 plan
For LTE to work on Simple you will need a phone that supports T-Mobile's 1700 Mhz AWS LTE band, also known as Band 4. That band is supported by all LTE capable T-Mobile branded phones as well as the unlocked Google Nexus 5, Motorola Moto X, iPhone 5C and 5S and a few other phones.
Simple joins the brand T-Mobile MVNO 35orLess in being the first of the magenta operator's reseller's to have LTE. I hope that we will soon seen it on the other T-Mobile MVNOs.