I think you may want to give us little more details:
- You did not write what kind of software you are using to control the LimeSDR board: LimeSuite, custom SoapySDR-based app, or maybe something different, like UHD-based app or some framework like GNURadio?
- Also, did you try to force TDD mode manually or just set the same TX and RX center frequency, hoping that the driver will enable TDD automatically?
As for the latter, I believe at the moment it has to be set manually, and some tips how to do that were given in this topic. That being said, I tried to force using the same PLL to drive both mixers via SoapySDR API in the way which is described there, but with no luck so far. Unfortunately, at the moment I got no time for further investigation, but got it on my TODO list.