A splitter is probably not a good idea at these frequencies. In a perfect world, you will lose 3 dB
(1/2) of your transmit power to the antenna and the same loss on receive. At these frequencies the
layout and components used in building said splitter become critical to insure each port sees the
desired impedance. As mentioned, the output power from the gateway may very well damage the
front-end of the RX dongle, or overdrive it to the point of being useless.
A directional coupler specifies a loss at the reflected port, usually 20 or 30 dB (a lot). Calibrating such a system will require real test equipment to produce any meaningful results. Personally, I would use dedicated RX and TX antennas for what you’re trying to accomplish. The TV splitter and
‘taps’ referenced will almost definitely work at ~900 MHz.