As the silo walls are metallic, locating ~800 MHz antennas inside means you will have a resonator. The wave mode depends on : resonator dimensions in wavelengths, locating the antennas can determine optimum points to operate the resonator in a "whispering-gallery mode", but the material in the silo and its electrical properties will affect the system quite significantly.
Take a textbook on microwave resonators to learn the details then try to figure out if it makes sense to go on.
Do you want only to communicate between selected points, or, do you want to use the UHF signal to indicate material height, or its properties?
You can theorize to start but experiments are needed to find out the results. A pretty interesting application!
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To find grain level in the silo, I would prefer to use weight sensors (tensometers) on silo external supports. You can use an ultrasonic sensor or a lossy cable hanging from the top, allowing to measure cable capacitance indicating the height.
Using your ~800 MHz probing is possible but a quite deep study is needed.