Power over coax filter selection
Reading between the lines, it sounds like you are trying to send a serial data stream (RS-232-ish). That requires a DC connection to the signal source, so this is just not going to work. Putting that aside, I will still answer your question.
The 115200 signal contains frequency content at 115200 and higher harmonics. If you want to pass DC, but block 115200, you want to place your filter cutoff frequency much lower than 115200. Much lower means by a multiple of 10x or more. This will insure the filter attenuation is substantial at 115200. In this case, since there isn't really a lower limit, I would suggest going 100x lower for your cutoff frequency. That will give you an cutoff of 1152 Hz as your target.
The rolloff from an LC filter is 40dB per decade (power). So by going 100 lower (two decades) you should be getting close to 80 dB of power attenuation. Hopefully that will be enough. If not, you can move the cutoff lower.
As far as the DC problem goes, one option might be to use a simple RF encoding scheme to send your signal over the wire as an AC signal. A simple on-off keying or two frequency FSK signal. Then you can still use RS232. You will just need to find a modulator/demodulator IC and put one on either end.