Your question is not about Wasabi but about Umbrel and how to expose the Bitcoin Node RPC. I don't know how your infrastructure is configured but if you have your node's RPC enabled and the RPC port is opened in your Umbrel then you should be able to connect to it and you could test the connection using bitcoin-cli. My suggest to ask to Umbrel team/community how to expose the RPC.

Finally, Wasabi requires compact filters from your node if you want to synchronize and operate without using any external filters' providers. That means that you have to enable the creation of compact filters in your node.