The normal 250K EB JR volume pedal won't work correctly; you need the 25K model built for active instruments (model 6181).
You also need an "insert" cable (1/4" TRS male > 2 mono 1/4" male). Run the TRS end into the 11R, the two monos into the in and out of the EB. If it doesn't work, swap the in and out on the pedal (then mark which is which
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IIRC, some of the newer EB JRs have an "expression" output, which, I believe is a TRS connector. If that's right, you only need a long TRS>TRS cable into the 11R's expression jack.