This section of Assassin's Creed: Odyssey walkthrough includes the description of how to reach the Tomb of Eteokles and find the Ancient Stele.
Tomb of Eteokles
Tomb location: Attika.
Walkthrough: Enter the tomb, continue forward, and smash the wall. Continue along the corridor and watch out for traps and snakes.
When you are inside the chamber with the sarcophagus go to another passage and continue downwards. You will reach a room with two wooden shelves you can move. Kill the snakes and pull the first one, to reveal a crack.
Now move the second shelf inside the crack to reveal a passage. Continue forward - watch out for traps and snakes.
Open the chest and discover the Ancient Stela.
Move another shelf and take the shortcut to reach the exit - you have successfully completed another tomb.
Mycenaen Tomb of Ajax
Tomb location: Attika, Isle of Salamis.
Walkthrough: Enter the tomb and continue forward. Smash the wall. Continue downward along the corridor and then turn left.
Continue forward - after you avoid the trap at the ramification, turn left. Kill the snakes and open the chest with the treasure.
Return to the corridor and run to the other side - you will reach a chamber with three cracks. Enter the one to your right (looking from the entrance) and search the chest.
Go back and head to the crack Ahead. Continue forward and avoid any traps/snakes. After a while, you will reach a room with a hole in the floor. Jump down to find the chest and the Ancient Stela.
Move the wooden shelf and go to the other side. Turn left and run towards the exit along the previously visited corridors.