Hi yobo, i would advise starting off gently and not using the erection maintaining rings until you are comfortable with the pump. Due to your surgery etc, its likely that you may have had some penile shrinkage due to non-use , so if you use the pump daily for 30 minutes as an exerciser, this will recondition the penile tissue and encourage increased blood flow. When you start using the erection rings, select the largest one until you find the ring that keeps the erection in place when the pump is removed. You can use 2 rings if necessary. With regular use, 40-50% of men who use penis pumps for ED are able to regain erections without using it but because of your surgery, eventhough it was probably 'nerve sparing' the chances of this will be lower. But what you can do is to use the viagra or similar pill, when you pump and this will increase your chances of the erection staying without having to use the ring.
I hope the above is helpful and just let us know how it goes!
kind regards
Steve