Seems there's alot of misunderstanding on how portals work or how to utilize their function. Here's a guide hopefully it's understandable.

1. When one portal is built the second portal built will automatically connect to it. When a third and fourth portal are built they will automatically connect to each other just like how portal one and two did and so forth and so on.

2. You can tag portals therefore linking two portals with the same name ie Swamp or Home. These are case sensitive so remember your spelling and grammar.

3. You can either make new portal after new portal after new portal and set them as a permanent portal hub of never changing the tags on the pair of portals or you can have a Network by swapping tags around on a portal to travel to other tagged portals placed around the world.

4. On the subject of changing portal tags. If a tag is changed on a portal, say going to Swamp and instead now going to Plains after the tag is changed the portal does not immediately change it's location. If you jump through the now Plains portal straight away it will instead take your to the initial tag of Swamp.

Why? The portal hasn't rebooted as you are standing next to the portal while the wormhole has already engaged to Swamp. Walk away from the portal so the wormhole can disengage, you can physically see the portal "fire" extinguish, then reapproach
the portal and it will engage a wormhole to the tag you set on it, in this case the Plains.

5. Changing a tag on an engaged wormhole can be utilised to change the tags on a pair of portals for instance I will engage the Swamp portal, change the tag to Home101 jump through the portal and now I'll change the second Swamp portal to Home101 which has now relinked them with new tags.

6. Another use for changing engaged portal tags is exploration. Say you have an untagged portal at base that you can connect to with a portal when out exploring. Once you have placed the untagged portal it will link with the untagged portal at base. Before jumping through change the tag to, for example, North and then jump through. Once back at base you can either set another permanent portal up to connect to North or you can change the tag of the network portal. Go back through your linked North portal and now you can keep exploring and you still have a unlinked, untagged portal back at base to connect with while your out and about.

I hope this has been a helpful guide to portals in Valheim. Also portals don't work underwater, you can enter portals from both sides but you always exit on the rune side, which can result in some interesting building opportunities fyi.