Underflows are not surprising when trying to stream data in real-time from MATLAB/Simulink to the SDR hardware. Either MATLAB may not keep up with the required sample rate on the hardware, or the Ethernet link between your host PC and the board may not be fast enough. Make sure you use a dedicated network card, or USB3 dongle on your PC.
You can use the transmitRepeat method to work around these limitations. This loads a set of samples onto the board and transmits it repeatedly onto the air. Please see this answer for futher details.