The only difference between the _RAK4631-R_ (RUI3) and the _RAK4631_ (Arduino) is the bootloader it is shipped with - the hardware is the same.
Meshtastic requires the Arduino bootloader on RAK WisBlock NRF52-based boards. The process of converting the bootloader only needs to be performed once.
This conversion requires the use of either a [DAPLink](https://daplink.io/) or [J-Link](https://www.segger.com/products/debug-probes/j-link/). The most reasonably priced and available is the [RAKDAP1](https://store.rakwireless.com/products/daplink-tool).