How to force Mikrotik to recognize imported private key?

I know that I can generate a CA and the rest on the Mikrotik. But, signing takes 3-5 minutes, which is horrible.

If I create the same certificate + key with OpenSSL and import it, Mikrotik is not able to see the private key. I have only AT flags, no ‘K’?

Why is that happening?

This works without issues if done right. Can you give more detail about your commands and generated files?