In current versions it is not a requirement, nor is there any documentation I have seen recommending to have separate subnets between LAN and WAN. In the past there were issues if LAN and WAN shared a subnet. I've deployed RecoverPoint both ways and I don't see any difference operationally. My preference is a separate subnet for WAN for clarity and troubleshooting.
If you are using a virtual RPA, the ISCSI interfaces are required to be on separate subnets as one would expect.