If you missed, the testicles will have shot back up into the body and they will continue to develop either just under the skin or right in the abdomen.
The heat from the body can render the sperm less fertile, but it is absolutely not guaranteed!
Can you catch them now? At 6 days after ringing, you should still be able to feel two testicles in the scrotum if done correctly.
If you were regularly doing it with a larger breed, I'd have thought you would have noticed at the time if you didn't get both peas in the pod?!
With Shetlands I would have thought that if testosterone is still being produced (if not castrated properly) the rams would likely develop normal horns rather than weedy ones, I can usually tell our Hebridean ram lambs and wethers apart before weaning time, and ewes are horned in this breed, so I would think (though someone more experienced with the breed could correct me) that wether Shetlands, like Shetland ewes would develop no horns if you got the ringing correct.