Well you can create an account at UBI even without owning the sim, but as far as registering SH5 proper, that has to be done upon it's first time launch I beleive. At least thats how I did this, and was prompted to do so during the launch, though I dont remember excatly at what point.
But I had already created and account at UBIsoft even before installing the game, so upon install and subsequent starting up the first time I looged in and check the "remember me" box and then I think it was after this that I was prompted to register SH5 and the keycode was then required.

But as I read your results here, you'll have to resolve your network coonection issues before this can take place. And when it comes to connection problems they can be due to many causes, and I'd get in touch with UBI support on this. They can walk you though the tests and procedures to see what is the problem. And trust me they will help, I've delt with them on the phone myself and they will help you even with things that are not related to the software itself but of your computer, and your network configuration.

Tanks a lot!:DL