Which knot should I use.

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I have an old Mitchell reel. I am trying to set it up for Eleven Mile. My problem is the spool of Spiderwire that I have wont fill the reel, so I put Mono line on first & now trying to connect the Spiderwire to the mono. Witch knot should I use? I tried the Double fishermans knot & it didn't work. Please someone help me please.