

Remind them every, single, turn.
“Really? Your dagger? Not your cantrip?”
“Oh yeah, I always forget about those”
If they prefer to use it for thematic or aesthetic reasons, they’ll tell you and the mystery will be solved. Maybe there’s a class that does what they want and you can push them towards it. Or maybe they really are just that forgetful and they just need to be reminded every turn. Consider giving them a character sheet that more obviously shows what they can do. Action cards, spell cards, stuff like that might help too. But ultimately, just don’t let them make a dagger attack. Just stop them and present the better option EVERY TIME. You’ll learn one way or another what they want because they’ll go with it or resist it
And I’m not necessarily saying to find a new group (although it seems like you’ll have no choice since you’re moving). I’m just saying, I cant imagine how you could keep playing with this person and no one at the table is making suggestions on how to play their turns. My players strategize about each others’ turns constantly. It’s a bit meta-gamey, but they’re newer and don’t know all the mechanics well so I never stop them. In fact, I try to help them find the course of action that will make them feel the coolest or the most useful without outright telling them what to do
I’ve got to recommend Monster Hunter Freedom Unite. One of the most legendary games in the series for its sheer amount of content. It might feel a bit dated if you’ve played World or Rise, but still good