I've try it. It seems that the N2251 receive something but I don't have any image. Just black screen.
I am checking with the product manager for more info. We do publish a compatibility guide for current models, but this combination is not covered. Sharing a link here for others:https://www.amx.com/en-US/site_elements/product-guide-n-series-stream-compatibility-guide
Product management reports "There is no compatibility between the N2121 and the N2251"