How can I use a portal to travel to specific locations in the new level?

I suspect that it makes no difference whether I work with a blueprint_actor(portal_x) or with the trigger_volume

No difference.

Options are passed as: key1=value1?key=value2?key3=value3 and so on.
In your case, since you just need to pass a hard coded vector you can just do it like:

So screenshoot A or B, you don’t need the Append node, Just write the the options directly in the node.

In Screenshoot C, since the first thing you want to do is to move your character to the right location, connect the BeginPlay to Set Actor Location.
The select node that I used is required since I didn’t pass any options for the first level (when you start the game) and I didn’t placed any Player Start. Without the select node, the player starts at (0,0,0).
In you case, for example, you can place a Player Start node where you want the player to start the game and then:

So, now the player starts the game (level 1) at the location you want, and for the rest of the levels, the player will start where the options in Open Level tells the game mode to.

PS: If you pass the value thought the options, then you don’t need to use Game instance for this.

PS: this method works, but like I said before, a more correct approach is where you spawn directly where you want. But that’s not required for now. The important is to have something working.