Push it back forward onto the stop and note the position of the upper pulley with respect to the sprockets, especially larger ones. I suspect it's riding super close or maybe touching (through the chain) in which case the sprocket will push the RD back until it clears.
It's possible the largest sprocket is too big for the RD, or possibly the chain is too long. Check the latter by shifting to a typical low gear combo, and pulling the lower loop forward (while holding the crank) and see if the upper pulley drops away from the sprocket, and the Rd swings forward. If that seems promising, remeasure the chain by going to the big/big combo and seeing how much you can safely shorten the chain. (minimum length is big/big, with some room for the idler cage to give more).
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Chain-L site
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