Having problems linking texture parameters from blueprint to material node

I am working on an effect that involves subtracting pixels between successive frames (as a postprocessing effect), and before I fully construct the logic for that, I am trying to get my blueprint and the postprocess material to communicate because the image frames would have to be transferred.

My level blueprint looks as follows:

BeginPlay - set up two render targets linked to a material instance

Tick - render to either T1 or T2, associate with parameters Tee1 and Tee2. Based on which one is currently active, my idea is to treat the other as “previous frame”.

Postprocess material node. I expect the textures from the blueprint to show up in Tee1 and Tee2, but all I see are the default (placeholder) textures.

How can I get this connection to work between the parameters in the blueprint and the param2D in the material node?