I would like to know what this circuit is
The device is a 4g cellular modem/router, and the chip is very likely an RF switch to allow selection between the internal antenna (visible on the PCB) and an external antenna jack (visible at the lower right corner of the photo).
Being a switch external to the modem, its purpose is to support basic &quot;diversity&quot; reception. The modem can compare signal strength and quality between the internal and external antenna and choose the one providing the best signal.
The 4G standard also supports &quot;MIMO&quot; antenna configurations which are more advanced, using the two antennas simultaneously to increase data throughput. However, I believe this requires connecting both antennas directly to the modem, not switching between them externally, so I don't think your unit has true MIMO.
Identifying the chip remains &quot;an exercise for the reader&quot;. At a major distributor (e.g. Digikey) website, you'll find the RF Switches category. Choose a manufacturer (there will be less than a dozen), look for a chip that's physically similar, pull up the datasheet, and look at the product coding. Usually the first set of digits is a short form of their product number (H51 in your case) and the second is a manufacturing lot (RFA). Good luck!