It's OK to use the IP address. However, the "Negotiation failed. Server signature is not valid." error suggests that this is not a host name or IP issue. The connection was successful - the failure occurred while negotiating a secure SSH session with the server. A possible reason is an incompatibility between the client and the server - we can most likely fix this with a bit of your help.
Would it be possible to create a communication log (as described at https://www.rebex.net/kb/logging/
) and post it here or mail it to email@example.com for analysis?