According to the manual, the router should come with 3 high-gain Wi-Fi antennas with reverse SMA connectors: https://cradlepoint.com/sites/default/files/cradlepoint_mbr1400_manual_5.pdf
These antennas are rated 5dBi, and only use 2.4GHz frequency range.
You can maybe look at some 12dBi antennas like this set: http://www.amazon.com/gp/product/B00KQ0EN2A/ref=s9_simh_hd_bw_b3I4TAx_p147_d0_i4?pf_rd_m=ATVPDKIKX0DER&pf_rd_s=merchandised-search-2&pf_rd_r=0REZBZBP2MH6HP5XEK70&pf_rd_t=101&pf_rd_p=2223046082&pf_rd_i=3015438011
They are long. I like 9 stacked bricks.