How do I convert a sampled complex signal to a real signal?
So the point is that "classically", communications theory tends to be done in complex baseband, i.e. signals centered around 0 Hz, but not necessarily symmetric in spectrum.

When you want to represent such signals, you need complex values. When you want to transmit such signals over the air, you need to:


Convert them from digital to analog,
mix the I(nphase) and Q(uadrature) components with the carrier (say, a cosine) and a 90° shifted version of the carrier, respectively,
and add up the result.


Since I and Q themselves are just real values, the result of 3. is just a real signal.

Now, if you do the above in the order that I've written it in, then you need two DACs running in step at the baseband sampling rate, and you can produce any RF frequency that the carrier synthesizer can generate.

But, you don't have to do it in the order 1., 2. and last 3.: You can also first digitally upsample to a higher sampling rate, then multiply the complex baseband signal with a complex sinusoid of frequency $f_\text{IF}$ to move it up to an intermediate frequency, and just discard the imaginary part of the signal. Then, you DAC just that real signal. 

You could then use an LO of frequency $f_\text{carrier}-f_\text{IF}$ to move it to the final carrier frequency. But: 

The FL2K is just a DAC. So it doesn't have a mixer. So you can't do that.

However, every DAC produces images at multiples of the sampling rate (that's why you always need an analog reconstruction filter, which classically is a low-pass cutting off at half the sampling rate). So, if, say your IF is $f_\text{IF}=10\,\text{MHz}$ and your sampling rate is 50 MHz, then you can produce a signal at 210 MHz simply by not low-passing the output of the DAC to below 25 MHz, but bandpassing for say 200-220 MHz.