You just have to start small, and eventually you might get there. It was like that for me with Signal. I was able to get a couple of my more willing close contacts to begin using it. After a couple months, another began using it, and eventually after two to three years the vast majority, most whom I never tried convincing, are using the application actively.

Just keep trying! It took a long time, but I managed to move my whole direct family to signal.

You will probably have to install it for them.

Make a group chat.

Use it regularly.

But as it’s basically the same functionality, you won’t have to train them to use it. It’s worth it.

Drop it and just go with e-mail, postal mail, phone calls, and SMS.

I did, and found out who my real "friends" really are.

Play dumb, say WA is broken on your phone, ask then to install Signal to work around. Use Signal with whoever agrees

I'm not sure you've followed OP. It could well be the case that his workplace and school activities are communicated via WA. His "playing dumb" results in him cut-off from valuable information he actually needs.

This is correct. Most workplaces and educational institutions use WhatsApp to communicate with employees/students.

<in my country>

Also in mine, but it's not a global thing.