I think there are a few options available to you before going on to a penile prosthesis.
One thing you can do is to use a penis pump in combination with the Viagra or Cialis. To do this, you would take your viagra as usual and then engage in foreplay to get an erection. Then you use the pump to maximize the erection and then put the constriction ring at the base of the penis. If you have already tried that and it did not work, then you will likely require more invasive measure to achieve an erection.
In this case, the next step for you is to go back to your urologist to teach you how to use penile injection called trimix. This medication is injected into the penis by yourself or your partner with a very small needle just prior to intercourse to create an erection. This works very well for many men who cannot achieve an erection with Viagra etc.
Another option is the use a urethral suppository called MUSE that is inserted just prior to intercourse into the urethra
If this does not work, then the last resort is to have a surgical implantation of a penile prosthesis. This would allow you to have an erection on demand without any medications, however, it is a surgical procedure.
Here is a link for info on penile prosthesis:
Here is a link for penile injections:
Hope this helps and let me know if I can clarify anything.