I have this working with a Ardusimple SimpleRTK3B board that uses the Septentrino Mosiac X5 board.  I'm sending RTCM 3.0 messages (1006, 1008, 1074, 1084, 1230) to my own SNIP server running in my office and broadcasting over the internet.  I have a Dynamic DNS hostname pointing to me instead of a static IP.

In the FMX you have to go to AgRemote and adjust Port D to accept RTCM messages.  I believe I used the quick start guide from DigiFarm to configure it.   One thing to note......I'm not sure how this config works if your FMX doesn't have a radio in it.