Unable to create a proper PEM for Redis Desktop Manager

On Ubuntu, I have a problem to establish a connection to redis server using SSH tunnel **and** SSH key with Redis Desktop Manager (RDM). What are the symptoms? - I can connect to the server where redis is running using "plain" ssh and my id_rsa, - other utilities which use either the SSH agent or keys in .ssh can connect to this server and create tunnels (e.g. DB apps), - I can connect with RDM to redis servers using SSH tunnel **and** password (so the question is not a duplicate of https://unix.stackexchange.com/questions/256912/unable-to-establish-an-ssh-tunnel-using-redis-desktop-manager) ; but this is not a perfect solutions, because I would rather use private/public keys authorization, - I cannot convert keys in .ssh to a *working* PEM format required by RDM: any PEM files I've generated using different methods I googled are rejected by RDM with a message Connection: Disconnect on error: SSH Connection error(Authentication Error): Unable to extract public key from private key file: Unable to open private key file, - I tried entering either a path to id_rsa (~/.ssh/id_rsa) or just a path to a directory where my private key is stored (~/.ssh). So, does anyone have an idea how to properly convert my SSH keys to a PEM format RDM needs and accepts?