Ok maybe I was not clear and assumed far too many steps, but if you look at what is suggested in the above link and then search through the soapy API code and see if it has already been implemented:
Which it has:
And then check when, where, or if EnableSXTDD is being called by the soapy API:
And then you just need to make sure that your code does what is required before ::SetFrequency is used.
Configure same frequency for RX and TX and make sure that the following is true, which it should be unless you are doing something more complex.
abs (freq + cF_offset_nco -center) <= 0.1 Hz