This is probably a result of GoAccess being ambiguous on scheme and your browser is deciding to use WSS when it’s connected to the site over HTTPS. Connecting over HTTP will likewise result in a WS connection, which works over LAN.
Port 7890 is also probably not firewalled inside your internal network, but is it available externally? If you want, you can configure Caddy to listen on this port externally and proxy it to GoAccess internally, that’s one way about it.
There’s no magic setting to make this one just work. You need to decide where you want your index file to be publicly accessible, where you want the websocket to be publicly accessible, and you need to configure GoAccess to point a client (the browser) at the correct URI for the websocket. It’s entirely up to you as to how to organise things and whether, and where, you want Caddy to sit in between the client and the application.