If you take your Range Rover to a dealer or independent they’ll tell you that you need a new transmission (stock answer as they’re incapable of fixing it).
I think you need to find a transmission specialist who will start with the simplest/cheapest solutions first as they carry out their diagnosis. This may be something along the lines of :
1) Check fluid level
2) Check TCC solenoid valve filter for blockage
3) Test function of TCC solenoid valve (replace?)
4) Remove valve block, clean and check function of Reverse Lockout Valve
I think you need to bear in mind that, unless you’ve already had it replaced, it’s possible that the torque converter lock-up clutch lining has started to disintegrate (a common fault on the 5L40-E) and this has contaminated the transmission fluid and caused the solenoid/valve sticking problems. They should be able to get an idea of this from the state of the fluid.
If this still didn’t provide a solution the transmission would have to be removed and stripped to check the low & reverse clutch (plates, seals, etc.).