OK. I have good news and bad news. The good news - I was able to program my RB75's. The bad - It took 6 months of working with Retevis Support. Even then I had to BUY a new cable and have them send me a new device driver. With the new cable and the new driver the radio was programmable from both the Retevis software and chirp. I tried all the combinations (old cable / new driver, new cable / old driver). Nothing worked. I had to use the new cable and the new driver. Then everything worked as advertised.
All I can say is - don't let them off the hook. Stick with them even if they take a long time to reply.
BTW - I have a set of RB19P's. They work great but don't believe the hype. I have tried to program them with a normal USB C cable. No way. Have to use their cable.
Not sure Retevis is worth the hassle.