I'm gonna be super for real with you. If your friends won't download an extra app to message you on then they're not really that good of a friend. It's such a small inconvenience. Omg I have to hit this other button on my touchscreen instead and spend 5 minutes setting it up?

This is how I'd do it. You send a message to your friends on instagram and whatsapp with your signal info and say, "Hey, not gonna be checking [this app] often and won't have notifs on. Trying to be more responsible about my data, and social media use. If you need me or want to chat hit me up on Signal."

If their response to that is "Okay well I don't want to so I won't talk to you anymore." Then they're not a friend worth keeping around anyway. Real friends are the people who will wake up at 3am and come pick you up from the airport when your coming home. Because they actually care about you, and when you actually care about someone you do inconvenient things for them.

Now, if by "friend" you mean some person you've never met that you chat with online sometimes and send funny videos to or whatever? That's not really what I'd call a friend to begin with, and you'll just have to accept that relationship isn't very deep and it's totally normal for them not to want to do something just to talk to you. And in that case you have to decide if you'd rather stop talking to them, or stay on the other apps. That's entirely a you decision at that point.