I recommend you read the guide Portals 101 which Fzanco mentioned.

It explains a property of portals that has not been mentioned in this discussion. That is that if a portal connected to another is renamed, it will take a couple of seconds to connect to the portal matching the new name. If you step through the renamed portal before the new connection takes effect you will be teleported to the portal bearing the old name. In the time taken to teleport the renamed portal will have connected to the portal matching it's new name. You will be stuck at the old portal with no way to teleport back.

I discovered this by accident. I had to trek and sail back to the renamed portal. I called my portal a name unsuitable for use in mixed company. It was also to long to fit the number of characters Valheim allows for portal names.