USB slave is not enabled in current FriendlyWRT. It does seem to be enabled in armbian focal. I tested this by plugging the r2s into my linux workstation and monitoring the USB port with tshark and the usbmon kernel module. Booting from FriendlyWRT shows no packets on the micro USB port. Booting from armbian focal shows lots of control and data packets captured.
Which distribution are you running on your R2S?