Can't connect to my first server (other than the PRTG host)

I've been testing PRTG on the same server that hosts PRTG. Now, I'm ready to try adding another server in the same subnet but am getting the error PR015:

Connection could not be established (Port Error : RPC Server not accessible. - Host: <name-of-machine>, User: administrator, Password: **, Domain: ntlmdomain:<name-of-machine>) (code: PE015)

I've gone through pretty much everything on this page:

https://helpdesk.paessler.com/en/support/solutions/articles/76000062954-my-wmi-sensors-don-t-work-what-can-i-do

- checked firewall - checked WMI is running - checked to be sure username and password are same as on PRTG host

Now it's starting to get too time consuming. Not sure what else I should do before giving up. How can I isolate what is the cause of this problem? Thanks a lot for the help.