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/
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1. Link your iPad with PC where .pem file installed.
2. Open iTunes and click iPad -> File Sharing
3. Click 'Termius' from the list then you can put your '.pem' file at the right side.
4. Go to Termius and open keychain then '.pem' file will show up