I don't have any ports open and I get the message: "No incoming connections - check firewalls/routers". What does this impact on my node? Can I still connect to my node locally and use its RPC interface?
What does this impact on my node?
By not forwarding ports, you are preventing access for external nodes. Note however, nodes that you initiate a connection with can still sync from your node.
Can I still connect to my node locally and use its RPC interface?
Yes.