I was able to successfully setup a dedicated server using my backup PC and it works well. I had to go into my Xfinity App to setup the port forwarding to that specific PC, so you should have to do something similar I would imagine with your ISP. Also, if you have any other things running on the computer, check those too like Anti-Virus or Windows Defender.

I used this guide mainly: https://www.pcgamer.com/valheim-multiplayer-dedicated-server/

Also, I used this post to troubleshoot some things as needed: https://steamcommunity.com/app/892970/discussions/0/3103515720272633154/

Let me know what is going on and where you are having issue and hopefully I can help you out since I just did this over the last few days. The best thing about getting this way to work is that it's not costing me any monthly fee, so don't give up!