I have an Archer AX21 and already had an RE450 extender and created a One Mesh with those two.

Got another RE450 and during setup, it does not give me the option to select One Mesh. Even after it's setup, One Mesh doesn't show as an option. 
 

what gives?

Do I need to undo the Mesh and create again? It won't even pop up the new device as a One Mesh option in the router settings.