Care for one, care for everyone
It may not be obvious at first, but these two have a shared interest in looking after people. Cancer takes care of the ones nearby. Aquarius wants a better world for everybody. A Cancer can show an Aquarius what all those principles mean at the scale of one actual person, which might be a real discovery. An Aquarius can show a Cancer that there’s life outside the family’s old rules, and that people can love each other in less anxious ways.
Aquarius is also rather loyal, once you get past the cool exterior, and that can surprise a Cancer in a good way. They may not call, but they don’t leave.
Holding on, stepping back
When Cancer feels uncertain, they come closer: more texts, more questions, more looking after. When Aquarius feels crowded, they step back: more time alone or with friends, a reasonable voice. Each of those moves sets off the other. The more Cancer reaches, the more Aquarius retreats. Cancer ends up feeling like an unwanted burden, and Aquarius ends up feeling guilty and trapped. Both of them may be reading the other wrongly.
Their versions of a good evening can differ, too. Cancer has lit candles and made something special. Aquarius has invited over someone they met at a protest, because it didn’t occur to them that it would matter.
Agreeing on the distance
Talk about how much contact you each want, in numbers if that helps. “A text at lunch, and one evening a week that’s ours” is a kind of love letter to both of these signs. Aquarius can say what they feel, not just what they think, and can ask “Do you want a hug or a solution?” Cancer can have a life beyond the relationship, and let time apart mean nothing more than time apart.
There isn’t a lot of natural overlap here, so this depends a good deal on the rest of the chart. Is there a soft Moon on one side, or an airy one on the other? That can turn this into a very different story.