This actually should work. You just need to copy the content of the pem file to the private part of the key when copying it into Termius app in the Keychain section.
We’ll double-check it.
If you have any other problems please contact us here – http://support.termius.com/
What if I don't want to give all my users a copy of the PEM file? The login with the PEM file will allow root access. AWS supports public keys and has a method for public key support found at: https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/managing-users.html#create-user-account
How to make this work with Termius?