I see a lot of posts where a whole system restart fixes it.
It has been stated somewhere that Roon will start without an Internet connection, that has not been my experience
I am running 20 MB’s fibre, Windows 10, Ethernet to a tp link archer router ,
We suffer repeated power outages, my UPS’s cope with an orderly shut down but I tend not to leave them on to run the core as this ruins battery life.
My experience is that to get a first time start up I start each element in the chain in turn waiting until each element, Router etc, has fully started before moving on
I rarely have a problem. The time recently when I was impatient, Roon failed to start until the Internet was up. Once I had internet Roon started normally
Just my experience if it helps