Hi there,
I don't recommend using data to server. As soon as you turn it on your GPS will not be available to other services. I also don't use a pi or pyModbusTCP but Modbus is Modbus, it really doesn't matter your specific configuration using the pi. Thats the beauty of Modbus. With Modbus there is always at least one slave and always only one master. A slave only speaks when asked by a master. In this setup your pi is the Master and RUTX12 the slave. So the RUTX12 will not "send" the data on its own. The pi must ask and the RUTX12 will "reply".
First of all you have to know if you want to keep the connection open to the RUTX12 and poll regularly like once every second.
c = ModbusClient(host="[rutx12 IP here]", port=502, unit_id=1, auto_open=True)
Or if you wanna open the connection, get the data and close it afterwards.
c = ModbusClient(host="[rutx12 IP here]", port=502, unit_id=1 auto_open=True, auto_close=True)
Once the connection has been made you can ask for the data. Here is a list of registers for the RUTX12. The signal strength is register #4.
regs = c.read_holding_registers(4, 1)