Note that the connected game clients do all the heavy lifting and the server is mostly just managing communication between them so it's a pretty lightweight program. If you have an older computer sitting around the easiest setup is to install the server on that, give it a fixed local ip address and leave it running 24/7 wired directly to the router. With that setup you can tell the router that machine is your DMZ, the default destination for all incoming connections.

If you need to have the server running on your main device you'll need the more complex router settings shown in the guides. Eg: https://www.rockpapershotgun.com/valheim-dedicated-server-hosting-how-to-setup-dedicated-server-to-play-with-friends

Question on dedicated servers: Are there any that support the serverside simulation mod? That makes the server a lot less lightweight but could be a good performance boost for anyone connecting with older hardware.