This did it for me. One thing to keep in mind is that your eyes may not be perfectly level. once I realized this I tilted my head to compensate and saw it.

This is because you over-relaxed your gaze. You can practice this with the two dots in the first few seconds of the linked video. As the video explains, if you relax your gaze the two dots (o o) will become three (o o o — this is the level of relaxation you need to see the big plane). If you relax your gaze even more, those three dots will become 4, however, they must be equidistant (o o o o, not: o oo o). Once you’re able to relax your gaze on command to see 3 or 4 dots perfectly spaced out, you can then use that skill to see the big plane alone or the small plane over the big one.
There’s even another level of relaxation where the small plane is kind of split up into 3 separate pieces over an out of focus large plane. Using the dots example, that level of relaxation looks something like this: o o o o, it’s very hard to keep focused and my eyes feel strained after a few seconds.

it is there