There can be as many as 4 Elder register Stones in the world map.
When I landed I landed at a set of stones for the trophies and the glowing Red RhuneStone was nearby to them. Once you register it it will showup on your map where teh actual Alter is and where you need to burn 3 ancient seeds.
They can be inside buildings in the Meadowlands or Mountains anywhere really.
Have a look around the area where you first landed at the cermonial stones where we hang our trophies for the Boss's heads.
Workaround
Get your map seed number the one you see when you first start game its in a panel and is a long number copy it and place it in this World generator tick to show the bosses
https://valheim-map.world/

Edited for spelling.