tangle, knot, fasten, fetter
縺 centers on threads or cords becoming twisted and caught: tangling, knotting, or being fettered. The fastening sense is a related application where something is tied or bound.
縺 combines 糸 (thread) with 連 (link, connect), suggesting threads that become linked or tangled together. The exact historical development is uncertain, but the phono-semantic structure is plausible.
The left side is thread 糸, and the right side is 連, which suggests linking or connecting. Picture threads that are linked together so tightly that they become a tangled knot.
For レン, imagine a tangled knot that looks like a lens: lens -> レン, and the threads are so twisted they form a lens-shaped tangle.