Any insomniac network experts about who can help with this?

Having problem setting up Outlook365 email to local mail server.

Can connect wirelessly using hostname.com as server name – also works from phone. But using ethernet, hostname.com doesn't work and have to use local address (192.168.1.100) and this throws (understandable) SSL certificate error.

Prefer to use ethernet on PC, any idea where problem is, router port forwarding?