In addition to Group and Channel, the newer Godox/Flashpoint transmitters and receivers (both internal and external) have the ability to designate an ID number of 1-99 (plus "Off"). The older pieces of the system do not have the ability to use ID numbers. In order to use the older pieces with the newer ones, the ID number setting needs to be "Off" on the newer units. Even with the newer units, the ID numbers that are set on each unit must match each other or the other unit will ignore it.
If your X1R-C uses firmware version 18 made since January 10, 2018 then the following instructions allow you to set the "RF ID" on the X1R-C:
The customer can set RF ID. Operation: press the CH and GR button simultaneously and power it on to enter the RF ID setting interface. Short or long press the CH button to add the ID value while short or long press the GR button to decrease. Power off the flash trigger after 5s, and ID value can be set and saved. Then, power it on, and the ID value will affect RF reception.
To check what firmware version your X1R-C is running hold down the group button [GR] while turning the X1R-C on. The firmware version will be displayed as long as you hold down the [GR] button. When you release the [GR] button the firmware version will no longer be displayed and the channel and group number will be displayed.
There's a micro-USB port under the rubber flap on the left side of the X1R-C that will allow you to attach it to a computer to update the firmware using the Godox G1 Firmware updater software.