Reading MODBUS RTU slave device Registers via RS485

Based on the information you have provided the 2-pin port is unlikely to support RS485.

1. Since there is no mention of batteries in the remote control then the two wires are carrying both power and data. RS485 doesn't do than (in any configuration I've seen) and uses a differential pair, usually with a shield.
2. A 'normal thermostat' (bimetallic strip type) would not be detected on an RS485 network.

More than likely they have implemented their own protocol something like this.

^{simulate this circuit – Schematic created using CircuitLab}

Power is fed out to the remote through a voltage dropping resistor. Let's say that when the remote is not transmitting data that it loads DRC+ enough to pull the voltage down to 20 V. When it transmits data it loads the line even further by connecting a resistor across the wires by a switching transistor. In our example the voltage might drop to 10 V for a '1' and back to 20 V for a '0'. The low voltage may have to be high enough to continue powering the remote - hence the 10 V.

Connecting a regular thermostat would present 31 V on DRC+ when open and 0 V when closed. Additional circuitry must be employed to detect and handle these signals.

There is no need to compare to someone else's data capture. The Modbus spec is pretty clear about what makes a valid Modbus packet.

Actually there are two specs that apply in your case. The first is the Modbus application protocol spec. That tells you what needs to be in a Modbus packet as seen by the applications on the master and the slave. They refer to this packet as the PDU. Then you also need to read the Modbus over serial spec. That explains how PDUs are wrapped for transmission over a serial line. This adds a address byte at the start and two checksum bytes after the end of the application-level Modbus PDU.

You may want to get software that can generate packets for your device to resepond to, but looking at someone else's data capture isn't going to tell you anything the spec doesn't.