It’s not planar the mode you have to use to force 2d. Go to the osnap panel, at the bottom activate project mode, this mode will ensure that you are always picking points projected to CPlane.
Can you show a screenshot? It should work.
Also remember that what the tab key does is constraint/lock the direction, so you have to make sure that you are on the right point before you hit the tab key. That’s why I think is safer to use the project mode in your case.