I think sometimes you just can't introduce a wheel to adult rats. Female rats tend to use wheels more, but really, wheels just aren't as necessary and natural for rats as they are for, say, hamsters.
That being said, it is entirely possible (and even simple) to introduce adult rats to a wheel, it's just not guaranteed that they will understand, or want, to use it. The only successful way I've ever heard of introducing wheels to rats is using food. I think one way is dabbing some type of clingy liquid treat inside, as you're doing. I think it'd be simpler and more effective to just put the rat on the wheel, and use something solid, in your hand, to lure the rat though. You could also try spinning the wheel and "walking" your hand around on it to show your rats what it can do.
I got a wheel several months ago and tried to introduce it to my adult rats- I simply showed them how it worked by spinning it and left it in the cage. I came back an hour or so later to discover that one of my rats, Whiskers, (RIP ❤) was using it- but my other boy, Willow, I have never seen using the wheel. However, I'm confident he knows how to use it, having seen his brother using it, so I think rats are capable of easily understanding how to use wheels, but they may just think it's not very fun or using it- not the wheel itself- is scary.
P.S: Is the stealth wheel safe for rats? Doesn't it have a wire running area?